MATERIALS  OF  MACHINES 


BY 

ALBERT  W.  SMITH 

DIRECTOR  OP  SIBLEY  COLLEGE,  CORNELL  UNIVERSITY 


SECOND  EDITION,  REWRITTEN 

FIRST    THOUSAND 


NEW   YORK 

JOHN    WILEY    &    SONS,   INC. 

LONDON:  CHAPMAN  &  HALL,  LIMITED 

1914 


COPYRIGHT,  1902,  1914 

BY 

ALBERT  W.  SMITH 


Stanhope  flfress 

F.    H.GILSON   COMPANY 
BOSTON,  U.S.A. 


PREFACE  TO   SECOND  EDITION 


THE  object  of  this  book  is  to  supply  elementary 
knowledge  of  metallic  materials  used  in  the  construction 
and  operation  of  machines.  The  aim  has  been  to  bring 
together  a  group  of  correlated  facts,  to  state  them  clearly 
and  to  show  their  relation. 

The  book  is  in  two  parts :  the  first  part  deals  with  the 
manufacture  of  materials;  and  the  second  deals  with  the 
properties  of  materials.  Understanding  of  the  first  part 
is  believed  to  be  an  essential  preliminary  to  the  study  of 
the  second  part,  while  understanding  of  the  second  part 
is  very  desirable  for  those  who  design,  construct  and 
operate  machines. 

The  writer  wishes  to  express  his  great  obligation  to 
Professor  Herman  Diederichs  for  his  ever-ready  and 
efficient  help  on  part  first,  to  Professor  George  B. 
Upton  whose  researches  and  kind  suggestions  made  it 
possible  to  present  part  second  in  its  present  form,  and 
to  Professor  Heinrich  Ries  for  his  kindness  in  criticizing 
the  chapter  on  Refractory  Materials. 

Those  who  wish  to  follow  part  second  with  a  fuller 
and  more  scientific  treatment  are  referred  to  Professor 
Upton's  forth-coming  book  on  "  Materials  of  Construc- 
tion." 

A.  W.  S. 
ITHACA,  July  1914. 


iii 


TABLE   OF   CONTENTS 


CHAPTER  PAGE 

I.   PRELIMINARY  CONSIDERATION  OF  FUELS 1 

II.   ELECTRIC  FURNACES 26 

III.  REFRACTORY   MATERIALS 30 

IV.  OUTLINE  OF  THE  METALLURGY  OF  IRON  AND  STEEL..  37 
V.   OUTLINE  OF  THE  METALLURGY  OF  COPPER,  LEAD,  TIN, 

ZINC  AND  ALUMINUM 77 

VI.   TESTING  MATERIALS 91 

VII.   THE  EQUILIBRIUM  DIAGRAM  OF  IRON  AND  CARBON.  .  110 

VIII.   CAST  IRON ' 121 

IX.   STEEL 144 

X.   HEAT  TREATMENT  OF  STEEL 165 

XI.   NON-FERROUS  ALLOYS 177 

XII.  SELECTION  OF  MATERIALS  FOR  MACHINES 193 


MATERIALS  OF  MACHINES 


PART  FIRST  — MANUFACTURE 

CHAPTER  I 
PRELIMINARY  CONSIDERATION  OF  FUELS 

IN  general,  a  fuel  is  a  substance  which  evolves  heat 
while  uniting  chemically  with  oxygen.  The  fuels  ordi- 
narily used,  however,  depend  for  their  value  upon  the 
presence  of  carbon  or  hydrogen.  A  fuel  may  be  pure 
carbon  (solid),  pure  hydrogen  (gas)  or  combinations 
of  carbon  and  hydrogen  like  petroleum  (liquid).  Fuel 
may,  therefore,  be  solid,  liquid  or  gaseous. 

All  industrial  fuels  have  their  origin  in  plant  growth. 
A  growing  plant,  because  of  energy  received  from  the 
sun's  rays,  separates  carbon  dioxide  of  the  atmosphere  into 
its  constituents,  releasing  the  oxygen  and  storing  the  car- 
bon within  the  growing  plant  tissue;  it  also  stores  hydro- 
gen and  oxygen  derived  from  the  sap  so  that  the  resulting 
plant  fiber,  wood  or  stems  or  leaves  or  grasses,  consists  of 
a  combination  of  carbon,  hydrogen  and  oxygen. 

Wood  may  be  used  directly  as  fuel  or  may  be  converted 
into  charcoal.  Nature's  processes,  acting  through  long 
periods  of  time,  have  converted  plant  fiber  into  coal  or 
petroleum  or  natural  gas;  and  artificial  processes  produce 
fuel  gas  and  coke  from  coal,  and  produce  fuel  gas  from 
petroleum.  Hence,  the  potential  heat  energy  of  fuels  is 

1 


2  MATERIALS  OF  MACHINES 

really  energy  which  came  from  the  sun  and  was  stored 
through  the  agency  of  plant  growth. 

The  expression  "  complete  combustion"  means  the  com- 
bination of  a  fuel  element  with  that  amount  of  oxygen 
which  produces  the  most  stable  compound.  Thus,  com- 
plete combustion  of  carbon  produces  carbon  dioxide, 
C02;  and  complete  combustion  of  hydrogen  produces 
water,  H20. 

Combustion  of  unit  weight  of  any  fuel  produces  a 
definite  quantity  of  heat  which  is  called  its  calorific  power. 
Since  this  is  a  quantity  of  heat,  it  is  expressed  in  units  of 
heat  quantity;  in  this  case  British  thermal  units.* 

When  carbon  is  burned  with  restricted  oxygen  supply, 
the  gas  carbon  monoxide,  CO,  is  formed.  Under  proper 
conditions  of  temperature  and  with  adequate  oxygen  sup- 
ply, this  carbon  monoxide  unites  with  more  oxygen  forming 
carbon  dioxide,  C02,  and  evolving  heat.  Hence  carbon 
monoxide  is  a  gas  fuel. 

Calorific  powers  of  combustibles  from  experimental  de- 
terminations are  as  follows: 

Combustible  Calorific  power 

Carbon  burned  to  carbon  dioxide  (CO2)  14,540  B.t.u. 

Carbon  burned  to  carbon  monoxide  (CO)  4,500  B.t.u. 

Carbon  monoxide  burned  to  carbon  dioxide  (CO2)  4,300  B.t.u. 

Hydrogen  burned  to  water  (H2O)  62, 100  B.t.u. 
Marsh  gas  burned  to  carbon  dioxide  (CO2)  and 

water  (H2O)  23,500  f  B.t.u. 

*  A  British  thermal  unit  (abbreviation,  B.t.u.)  is  the  quantity  of 
heat  necessary  to  raise  the  temperature  of  one  pound  of  water  from 
32°  F.  to  212°  F.,  divided  by  180.  This  gives  the  average  quantity 
of  heat  per  degree  for  the  specified  temperature  limits. 

t  Since  one  pound  of  marsh  gas  is  composed  of  three-fourths 
pound  of  carbon  and  one-fourth  pound  of  hydrogen,  it  would  seem 
that  its  calorific  power  should  be  equal  to  f  X  14,540  +  i  X  62,100  = 
26,430  B.t.u.  instead  of  23,500  B.t.u.  The  difference,  2930  B.t.u.,  is 
energy  used  in  separating  the  carbon  and  hydrogen  of  the  marsh  gas. 


PRELIMINARY  CONSIDERATION   OF  FUELS          3 

The  calorific  power  of  carbon  monoxide  burned  to  car- 
bon dioxide  may  be  derived  from  the  calorific  powers  of 
carbon  burned  to  carbon  monoxide  and  carbon  burned  to 
carbon  dioxide  as  follows: 

The  carbon  in  one  pound  of  CO  equals,  from  the  relation 
of  atomic  weights,  if  =  f  pound.  In  burning  to  CO, 
this  f  pound  carbon  evolved  heat  =  4500  X  f  =  1928 
B.t.u.  If  it  had  been  burned  to  C02,  the  heat  evolved 
would  have  equaled  14,540  X  f  =  6231  B.t.u.  The  dif- 
ference between  these  two  heat  quantities  equals  the 
heat  that  would  be  evolved  by  burning  carbon  monoxide 
to  carbon  dioxide.  This  equals  6231  -  1928  =  4303 
B.t.u.  This  value  checks  closely  with  values  derived  from 
experiment. 

Temperatures  resulting  from  combustion.  —  When  a 
combustible  is  burned  and  the  heat  evolved  is  applied 
exclusively  to  the  products  of  combustion,  the  tempera- 
ture attained  depends  on  the  following  factors: 

1.  The  calorific  power  of  the  combustible. 

2.  The  nature,  relative  weights  and  the  specific  heats 

of  the  products  of  combustion. 

3.  The  quantity  of  air  supplied. 

4.  The  temperature  before  combustion  of  the  fuel  and 

the  air  that  supplies  oxygen  for  the  combustion. 

Factor  1  is  a  measure  of  the  quantity  of  heat  that  is 
available  for  application  to  the  combustion  products,  and 
with  a  given  quantity  of  a  given  material  to  heat,  the 
resulting  temperature  is  proportional  to  the  quantity  of 
heat  available. 

2.  The  temperature  of  combustion  depends  on  the 
nature  of  the  combustion  products;  this  may  be  illustrated 
as  follows :  If  the  fuel  is  hydrogen,  the  combustion  product 


4  MATERIALS  OF  MACHINES 

is  water;  the  water  must  be  vaporized,  and  the  heat  of 
vaporization  becomes  latent  and  hence  cannot  affect  tem- 
perature. This  is,  of  course,  not  true  when  there  is  no 
change  of  state  of  combustion  product  as  in  case  of  carbon 
burned  to  carbon  dioxide.  It  is  obvious  that  with  a  given 
quantity  of  heat  available,  the  temperature  increase  de- 
pends upon  the  weight  of  substance  heated,  and  upon  the 
quantity  of  heat  that  will  raise  unit  weight  of  the  substance 
through  a  temperature  range  of  one  degree;  that  is,  upon 
its  specific  heat. 

3.  This  factor  affects  the  result  because  any  excess  of  air 
increases  the  weight  of  matter  to  be  heated  by  a  given  heat 
quantity  with  corresponding  reduction  of  temperature. 

4.  This  factor  affects  the  result  because  with  1, 2  and  3 
specified  the  products  of  combustion  would  be  raised  in 
temperature  through  a  certain  range  and  the  final  tem- 
perature would  depend  on  the  temperature  of  the  sub- 
stance when  heating  began. 

To  find  the  temperature  produced  by  complete  com- 
bustion of  pure  carbon  without  excess  of  air. —  Chemically 
the  combustion  may  be  represented  as  follows : 

24      64          88 
C2  +  202  =  2C02. 

The  relative  weights  appear  above  the  symbols.  For 
every  24  weight  units  of  carbon  64  weight  units  of  oxygen 
must  be  supplied ;  hence  for  one  weight  unit  of  carbon  f  £ 
weight  units  of  oxygen  must  be  supplied.  But  the  oxygen 
is  supplied  from  the  air,  which  is  a  mixture  of  oxygen  and 
nitrogen  with  very  small  amounts  of  water  vapor,  carbon 
dioxide  and  other  gases.  For  the  present  purpose  all  may 
be  disregarded  except  nitrogen  and  oxygen  which  are 
present  with  close  approximation  as  follows:  nitrogen 
77  per  cent  by  weight  and  oxygen  23  per  cent  by  weight. 


PRELIMINARY  CONSIDERATION  OF  FUELS          5 

Hence,  to  supply  23  weight  units  of  oxygen,  100  weight 
units  of  air  are  necessary;  therefore,  it  takes  l£J-  weight 
units  of  air  to  supply  one  weight  unit  of  oxygen.  Hence, 
for  complete  combustion  of  one  pound  of  pure  carbon  the 
weight  of  air  required  equals 

If  X  W  =  11. 6  pounds. 

The  resulting  gases,  when  combustion  is  complete,  are 
nitrogen  and  carbon  dioxide.  The  nitrogen  takes  no  part 
in  the  combustion  and  the  weight  is  77  per  cent  of  the 
air  supplied  or  11.6  pounds  X  0.77  =  8.93  pounds.  For 
every  pound  of  carbon  (see  equation  above)  ff  =  3.66 
pounds  of  carbon  dioxide  are  produced. 

Summary.  —  One  pound  carbon  (solid)  burned  in  11.6 
pounds  air  (gas)  produces  8.93  pounds  nitrogen  (gas), 
and  3.66  pounds  carbon  dioxide  (gas). 

Heat  changes  during  this  combustion.  —  Assume  that 
the  air  supplied  and  the  carbon  are  at  a  temperature  of 
65°  F.  before  combustion.  Then,  taking  0°  F.  as  the 
heat  datum,  or  temperature  at  which  heat  begins  to  be 
considered,  the  air  would  bring  to  the  combustion  11.6  X 
65  X  0.237  =  178.7  B.t.u.  in  which  11.6  is  the  weight  of 
air,  65  is  the  temperature  range  above  the  heat  datum,  and 
0.237  is  the  specific  heat  of  air  at  constant  atmospheric 
pressure  and  at  a  temperature  of  65°  F.  The  heat  that 
one  pound  of  carbon  would  bring  to  the  combustion  equals 
1  X  65  X  0.24  =  156.6  B.t.u.  The  complete  combus- 
tion of  the  carbon  would  evolve  14,540  B.t.u.  Hence,  the 
total  heat  available  to  raise  the  temperature  of  the  prod- 
ucts of  combustion  above  0°  F.  equals  14,540  +  178.7  + 
15.6  =  14,734  B.t.u. 

This  heat  would  raise  8.93  pounds  nitrogen  and  3.66 
pounds  oxygen  to  some  temperature,  t°,  to  be  determined. 

The  mean  specific  heat  of  nitrogen,  with  the  temper- 


6  MATERIALS  OF   MACHINES 

ature  range  0°  to  4000°  F.,  is  0.2848  *  and  of  carbon  dioxide 
for  the  same  range  is  0.2867.* 

The  heat  absorbed  by  the  nitrogen  while  its  temperature 
is  raised  to  t°  F.  equals  8.93  X  t  X  0.2848  =  2.543  t;  the 
heat  absorbed  by  the  carbon  dioxide  while  its  temperature 
is  raised  to  t°  F.  equals  3.66  X  t  X  0.2867  =  1.049 1  B.t.u. 

Hence  t  (2.543  +  1.049)  =  14,734, 

whence  t°  =  4100°  F. 

The  temperature  produced  by  burning  carbon  monoxide 

gas  may  be  found  by  the  same  method.     This  chemical 
combination  is  represented  thus; 

56        32         88 
2CO  +  O2  =  2CO2. 

The  weight  of  air  per  pound  of  carbon  monoxide  equals 
ft  X  W-  =  2.48  pounds.  Of  this  air  77  per  cent,  or  1.9 
pounds  is  nitrogen;  the  resulting  carbon  dioxide  equals 
If  =  1.57  pounds. 

Summary. — One  pound  carbon  monoxide  (gas)  burned 
in  2.48  pounds  air  (gas)  produces  1.9  pounds  nitrogen 
(gas)  and  1.57  pounds  carbon  dioxide  (gas). 

As  before,  assume  0°  F.  as  a  heat  datum,  and  65°  F. 
as  the  temperature  of  the  fuel  and  air  supply  before  com- 
bustion. Then  the  air  would  bring  to  the  combustion 
2.48  X  65  X  0.237  =  38.2  B.t.u.  and  the  fuel  would 
bring  1  X  65  X  0.245  =  15.9  B.t.u.  The  heat  evolved 
by  the  combustion  (calorific  power  of  the  carbon  monoxide) 
=  4300  B.t.u.  Hence,  the  total  heat  available  to  raise 
the  temperature  of  the  products  of  combustion  above  0°  F. 
=  4300  +  38.2  +  15.9  -  4354  B.t.u. 

*  See  "  Experimental  Engineering, "  Carpenter  and  Diederichs, 
page  865. 


PRELIMINARY  CONSIDERATION  OF  FUELS    7 

The  mean  specific  heat  of  nitrogen  (0°  to  4000°  F.)  = 
0.2848,  and  of  carbon  dioxide  =  0.2867. 
40^4. 

Then '  -  (1.9X0.2848);  (1.57X0.2867)  =  4397°  * 

The  temperature  produced  by  the  combustion  of 
hydrogen  may  also  be  found :  The  combustion  is  repre- 
sented chemically  as  follows: 

4        32         36 
2  H2  +  02  =  2  H20. 

For  every  weight  unit  of  hydrogen  8  weight  units  of 
oxygen  must  be  supplied,  and  the  corresponding  weight 
of  air  =  Vs0  X  8  =  34.8  pounds.  Hence,  the  combustion 
of  one  pound  of  hydrogen  requires  34.8  pounds  of  air,  of 
which  77  per  cent,  or  26.8  pounds,  is  nitrogen.  The  water 
resulting  from  the  combustion  =  -\6-  =  9  pounds. 

Summary.  —  One  pound  hydrogen  (gas)  burned  in  34.8 
pounds  air  (gas)  produces  26.8  pounds  nitrogen  (gas)  and 
9  pounds  water  (superheated  vapor). 

The  water  produced  is  raised  in  temperature  to  212°  F.* 
and  converted  into  steam  which  is  superheated  to  the 
temperature  resulting  from  the  combustion. 

The  specific  heat  of  hydrogen  at  65°  F.        =  3.37. 

The  specific  heat  of  air  at  65°  F.  =  0.237. 

The  specific  heat  of  water  at  65°  F.  =  1.00. 

The  mean  specific  heat  of  nitrogen  from 

0°  F.  to  4000°  F.  =  0.2848 

The  mean  specific  heat  of  steam  from 
212°  F.  to  4000°  F.  =  0.6724. 

The  heat  that  is  available  to  raise  the  temperature  of 
the  products  of  combustion  may  be  found  as  follows: 

With  0°  F.  for  a  heat  datum,  and  with  65°  F.  as  the  tem- 
perature of  the  hydrogen  and  air  before  combustion,  one 

*  Assuming  the  combustion  to  occur  at  atmospheric  pressure. 


8  MATERIALS  OF  MACHINES 

pound  of  hydrogen  would  bring  heat  to  the  combustion 
equal  to  1  X  65  X  3.37  =  219  B.t.u.  The  air  would  bring 
heat  equal  to  34.8  X  65  X  0.237  =  536  B.t.u. ;  the  heat 
evolved  by  the  combustion  (calorific  power  of  hydrogen) 
=  62,100  B.t.u.  The  sum  of  these  values  =  62,755  B.t.u. 
This  heat,  however,  cannot  all  be  applied  to  raise  the  tem- 
perature of  the  products  of  combustion,  because  the  heat 
applied  to  vaporize  the  water  does  not  affect  temperature. 
This  heat  =  970  *  X  9  =  8,730  B.t.u.  Hence,  the  heat 
that  really  does  affect  temperature  equals  62,755  —  8730  = 
54,025  B.t.u.  This  heat  raises  the  temperature  of  9 
pounds  of  water  from  65°  F.  to  212°  F.;  it  also  raises  the 
temperature  of  9  pounds  of  steam  from  212°  F.  to  t°,  the 
final  temperature;  it  also  raises  the  temperature  of  26.8 
pounds  of  nitrogen  from  65°  F.  to  t°.  Hence,  the  following 
equation  may  be  written: 

54,025  =  (212  -  65)  9  +  (t  -  212)  (9  X  0.672) 

+  0-65)  (26.8X0.2848). 
Hence  13.68*  =  54,025  +  1282.6  +  495.95  -  1323 

and  '-TO-*882"* 

For  the  assumed  conditions,  then,  the  theoretical  tem- 
peratures produced  by  complete  combustion  are:  for 
carbon  4100°  F.;  for  carbon  monoxide  4397°  F.;  and  for 
hydrogen  3982°  F. 

It  may  seem  strange  that  the  combustion  of  carbon 
monoxide  —  which  is  partially  burned  carbon  —  should 
produce  a  higher  temperature  than  the  combustion  of  the 
original  carbon;  especially  in  view  of  the  fact  that  the  calo- 
rific power,  or  heat  produced  per  pound,  is  14,540  B.t.u. 
for  carbon,  and  only  4320  B.t.u.  for  carbon  monoxide. 
Inspection  of  the  illustrative  examples  given  above,  how- 

*  The  heat  of  evaporation  of  steam  at  atmospheric  pressure. 


PRELIMINARY  CONSIDERATION  OF  FUELS          9 

ever,  shows  that  while  the  heat  evolved  in  the  case  of 
carbon  monoxide  is  less,  the  weight  of  the  products  of 
combustion  is  less  in  greater  proportion,  and  hence  the 
resulting  temperature  is  higher. 

The  reasons  why  hydrogen,  with  its  high  calorific  power, 
produces  a  temperature  lower  than  either  of  the  other 
fuels,  are  the  greater  relative  weights  of  the  substances 
heated,  their  greater  heat  capacity,  and  the  absorption 
of  heat  for  the  vaporization  of  the  water  produced  by  the 
combustion  with  no  resulting  change  of  temperature. 

The  theoretical  temperatures  found  are  never  attained 
in  actual  combustion  for  the  following  reasons: 

1.  Combustion    is    seldom    complete;     finely    divided 
solid  fuel  falls  through  grates,  and  carbon  monoxide  and 
marsh  gas  often  escape  unburned  to  the  stack,  because  of 
low  temperature  or  insufficient  oxygen  supply;  hence,  the 
theoretical  quantity  of  heat  to  raise  the  temperature  is 
not  completely  evolved. 

2.  In  practice  an  excess  of  air  is  always  supplied  in 
the  effort  to  prevent  incomplete  combustion,  and  this 
increases  the  weight  of  the  gas  to  be  heated  and  thus 
reduces  the  resulting   temperature.     For   example,    the 
temperature  resulting  from  the  complete  combustion  of 
carbon,  with  conditions  as  in  the  example  on  page  4  and 
with  50  per  cent  excess  of  air,  is  about  3000°  F.  instead  of 
4100°  F.  with  no  air  excess;  with  100  per  cent  excess  the 
theoretical  temperature  is  about  2300°  F. 

3.  There  are  always  radiation  losses,  which  increase 
very  rapidly  with  increase  of  temperature;    while  these 
losses  may  be  much  reduced  by  careful  design  and  con- 
struction, and  by  the  use  of  heat  insulating  materials,  they 
cannot  be  reduced  to  zero. 

4.  Moisture,  which  is  usually  present  in  the  fuel  and 
in  the  air  supply,  absorbs  heat  while  it  is  heated,  vaporized 


10  MATERIALS  OF  MACHINES 

and  superheated,  and  this  heat  is  taken  away  from  the 
heat  that  is  available  to  raise  temperature. 

5.  There  is  also  a  limit  due  to  the  fact  that  at  high 
temperatures  dissociation  of  the  products  of  combustion 
may  occur;  this  may  be  explained  as  follows: 

In  a  space  like  an  ordinary  furnace  containing  carbon, 
oxygen,  carbon  monoxide  and  carbon  dioxide,  there  are 
probably  two  coexisting  tendencies;  one  for  carbon  and 
oxygen  to  unite,  and  another  for  the  combinations  of 
carbon  and  oxygen  to  separate.  At  a  given  temperature 
these  tendencies  will  be  in  equilibrium  when  a  certain 
proportion  exists  among  the  substances  present.  As  the 
temperature  changes,  however,  the  proportions  corre- 
sponding to  equilibrium  change.  When  temperature  rises 
in  a  space  containing  the  substances  specified  above  in 
equilibrium,  some  of  the  carbon  dioxide  will  dissociate 
in  order  to  restore  the  disturbed  equilibrium.  This  dis- 
sociation is  accompanied  by  absorption  of  heat  which 
tends  to  check  the  rise  in  temperature  and,  therefore, 
to  limit  the  temperature  of  combustion  by  the  disso- 
ciation of  combustion  products. 

It  is  impossible  in  the  present  state  of  knowledge  to 
state  the  proportions  and  temperatures  quantitatively,  but 
it  is  certain  that  in  ordinary  furnaces  burning  carbon  fuel 
a  temperature  of  3000°  F.  can  be  produced,  while  it  is 
probable  that  dissociation  of  C02  would  prevent  the 
temperature  rising  inuch  above  3500°  F. 

Similarly,  highly  superheated  steam  from  the  combus- 
tion of  hydrogen  would  dissociate  with  rising  temperature 
to  restore  equilibrium  and  would  thus  limit  the  tempera- 
ture of  combustion  of  hydrogen  to  some  value  less  than  the 
theoretical  value  derived  above. 

In  engineering  and  metallurgical  processes  it  is  often 
required  to  produce  temperatures  higher  than  3000°  F. 


PRELIMINARY  CONSIDERATION  OF  FUELS   11 

while  guarding  against  incomplete  combustion  of  carbon 
with  a  50  per  cent  (or  greater)  excess  of  air,  or  while  using 
a  gas  fuel  diluted  with  a  large  proportion  of  nitrogen. 
This  can  be  accomplished,  up  to  the  limit  set  by  disso- 
ciation, by  preheating  the  air  supply,  and  the  fuel  also, 
if  it  is  gas  or  vapor. 

With  the  method  of  computation  used  above  it  is  found 
that  in  case  of  complete  combustion  of  carbon  with  50 
per  cent  excess  of  air,  the  preheating  of  the  air  supply  to 
1200°  F.  increases  the  temperature  of  combustion  from 
about  3000°  F.  to  about  3900°  F.;  while  with  100  per 
cent  air  excess  and  preheating  to  1200°  F.  the  theoretical 
temperature  is  about  3300°  F.  If  the  preheating  is  in- 
creased to  1500°  F.  with  50  per  cent  air  excess,  the  resulting 
theoretical  temperature  becomes  4200°  F.,  and  with  100 
per  cent  air  excess  it  becomes  3600°  F.  Of  course,  these 
values  would  be  somewhat  reduced  by  radiation  loss. 
Since  dissociation  (see  page  10)  sets  a  probable  limit  at 
about  3500°  F.  it  would  seem,  from  the  theoretical  temper- 
atures just  given,  that  preheating  of  air  to  produce  high 
final  temperature  might  be  overdone. 

If  pure  oxygen  were  used  instead  of  air  for  the  support 
of  combustion  of  carbon,  the  resulting  theoretical  tem- 
peratures would  be  higher,  since  there  would  be  no  nitrogen 
to  heat.  This  temperature  cannot  be  computed  because 
of  the  uncertainty  as  to  the  value  of  the  specific  heat  of 
carbon  dioxide  at  very  high  temperatures.  But  in  this 
case,  as  in  those  previously  considered,  the  dissociation 
limit  would  be  met,  though  its  value  might  be  changed 
because  of  the  different  composition  of  the  products  of 
combustion. 

It  follows  that  if  temperatures  much  above  3500°  F. 
are  required,  they  must  be  produced  by  other  means  than 
the  burning  of  carbon  fuel. 


12  MATERIALS  OF  MACHINES 

There  are  many  other  substances  in  nature  which, 
when  they  combine  with  oxygen,  produce  temperatures 
higher  than  those  that  result  from  the  burning  of  carbon 
or  hydrogen.  Two  such  substances,  silicon  and  alu- 
minum, will  be  briefly  considered  for  illustration. 

Silicon.  —  In  the  Bessemer  process,*  silicon,  which  con- 
stitutes only  from  2  to  5  per  cent  of  the  charge  at  the 
beginning  of  the  "blow,"  is  burned  to  silica,  SiO2,  and 
chiefly  as  a  result  of  the  heat  thus  evolved,  the  entire 
charge  is  raised  in  temperature  from  about  2500°  F.  to 
about  3500°  F.,  at  which  the  nearly  pure  iron  is  held  in 
a  fluid  state.  Obviously,  if  the  silicon  were  present  in 
larger  proportion,  its  burning  would  produce  a  higher 
temperature,  if  the  temperature  were  not  reached  at 
which  vaporization  of  the  silica  or  the  iron  would  estab- 
lish a  temperature  limit. 

Aluminum.  —  Finely  divided  aluminum  and  iron  oxide 
are  intimately  mixed,  the  mixture  being  called  "  ther- 
mit," and  when  this  mixture  is  ignited  the  oxygen  of  the 
iron  oxide  goes  over  to  the  aluminum,  forming  alumina, 
A1203,  and  leaving  pure  iron.  The  chemical  changes 
occur  rapidly  and  very  vigorously  and  the  resulting  tem- 
perature is  said  to  be  about  5000°  F.  At  this  temperature 
the  alumina  may  be  drawn  off  as  a  molten  slag  leaving 
the  pure  iron  in  a  very  fluid  state.  Sometimes  the  prod- 
uct of  this  process  is  used  to  mend  cracked  castings. 
The  hot  liquid  iron  from  the  process  is  allowed  to  run 
into  the  space  between  the  cracked  surfaces,  the  metal 
of  these  surfaces  is  melted,  the  space  is  filled  with  molten 
iron,  which  cools  and  solidifies,  and  the  cracked  surfaces 
are  joined. 

This  process  is  also  applied  to  the  production  of  metals 
like  tungsten  and  chromium,  in  a  very  pure  state,  from 
*  See  page  60. 


PRELIMINARY  CONSIDERATION  OF  FUELS   13 

their  oxides.  The  metallic  oxide  is  mixed  with  pure 
aluminum,  both  finely  divided,  and  the  mixture  is  ig- 
nited; the  products  are  pure  metal  and  aluminum  oxide, 
the  latter  being  removed  as  slag. 

The  use  of  aluminum  as  a  fuel  is  only  justified  by  the 
production  of  exceptional  results,  because  the  fuel  must  be 
produced  by  an  artificial  and  costly  process. 

Nature  does  not  produce  silicon  and  aluminum  fuel; 
nature's  processes  have  produced  vast  quantities  of 
silica,  Si02,  and  alumina,  A12O3,  in  which  the  combination 
with  oxygen  and  evolution  of  heat  has  already  occurred; 
that  is,  they  are  fuels  that  have  been  burned.  This  is 
true  of  almost  all  substances  that  might  be  used  as  fuel; 
in  fact,  it  is  true  of  carbon  and  hydrogen  which  occur 
in  nature  combined  with  oxygen  as  carbon  dioxide, 
mechanically  mixed  with  the  air,  and  as  water  in  its 
well-known  distribution.  But  the  energy  of  the  sun's 
rays  through  the  agency  of  plant  growth  is  continually 
pulling  away  carbon  from  the  oxygen  of  the  carbon  dioxide 
of  the  air  and  storing  it  with  hydrogen  and  oxygen  from 
the  sap  in  the  products  of  plant  growth.  In  the  past, 
these  have  been  converted  by  nature's  processes  into 
coal,  petroleum  and  natural  gas  and  stored  underground. 

Solid  fuels  may  be  classified  as: 

(a)    Raw  fuels,  such  as  coal  and  wood; 

(6)    Artificial  fuels,  such  as  coke  and  charcoal. 

Raw  fuels.  —  Coal  is  often  classified  as  follows: 


Coal 


Lignite. 

Bituminous  coal. 
Anthracite  coal. 


Plant  tissue  is  really  converted  into  coal  by  gradual 
change;    hence,  each  division  of  the  classification  covers 


14 


MATERIALS  OF  MACHINES 


a  wide  range  and  blends  into  the  others.     Description 
of  coals  is  unnecessary  here. 

The  following  table  of  percentage  compositions  shows 
the  chemical  changes  which  occur  while  plant  tissue  or 
woody  fiber  is  changed  to  anthracite  coal. 


Fuel 

Carbon 

H  and  O  in 
proportion  to 
form  water 

H  available  for 
combustion 

Wood  

48.5 

50  9 

0.6 

Peat 

'59  4 

39  0 

1  6 

Lignite  

65.0 

33.0 

2.0 

Bituminous  coal  

78.0 

19.0 

2.8 

Anthracite  coal. 

94  0 

4  0 

2  4 

During  this  change,  the  percentage  of  available  combus- 
tible increases,  and  the  percentage  of  water  to  absorb 
heat  decreases;  hence,  the  temperature  of  combustion 
increases. 

Wood.  —  According  to  Professor  Thorpe,*  woody 
tissue,  when  freed  from  soluble  and  other  foreign  matter, 
has  a  percentage  composition  as  follows:  carbon,  48.5; 
hydrogen,  6.2;  oxygen,  45.3.  Since  eight  parts  by 
weight  of  oxygen  unite  in  combustion  with  one  part  of 
hydrogen,  it  follows  that  if  the  percentage  of  hydrogen 
present  were  45.3  -s-  8  =  5.6+,  the  oxygen  and  hydrogen 
would  be  present  in  just  the  right  proportion  to  form 
water,  and  no  hydrogen  would  be  available  for  the  evo- 
lution of  heat.  The  amount  of  hydrogen  really  present 
is  6.2  per  cent;  and  only  the  difference,  6.2  —  5.6  =  0.6 
per  cent  of  hydrogen  is  available.  This  is  practically 
negligible.  Only  48  per  cent  of  pure  woody  tissue,  there- 
fore, is  available  for  fuel.  The  temperature  of  combus- 

*  "Coal:  Its  History  and  Uses,"  pp.  164-165.  Edited  by 
Professor  Thorpe.  Published  by  Macmillan  &  Co. 


PRELIMINARY  CONSIDERATION  OF  FUELS       15 

tion  is  low  for  this  reason,  and  also  because  the  water 
resulting  from  the  breaking  up  of  the  woody  tissue,  and 
that  present  as  moisture,  must  be  vaporized  with  absorp- 
tion of  heat  unaccompanied  by  rise  in  temperature. 
Therefore  wood  cannot  be  used  directly  as  a  fuel  for  the 
production  of  very  high  temperatures. 

Artificial  fuels.  Coke.  —  Bituminous  coal,  as  shown 
in  the  foregoing  table,  contains  carbon,  hydrogen  and 
oxygen.  There  is  also  a  small  amount  of  nitrogen  present. 

When  this  coal  is  highly  heated  in  a  closed  retort, 
destructive  distillation  takes  place.  The  products  of 
this  process  may  vary  with  the  time  occupied,  the  tem- 
perature, the  quality  of  the  coal  an^d  other  conditions, 
but  in  general  are  as  follows : 

(a)  Combinations  of  hydrogen  and  carbon  in  a  very 
wide  range  of  proportions,  resulting  in  solid,  liquid  and 
gaseous  hydrocarbons. 

(6)  Combination  of  hydrogen  and  nitrogen  into  am- 
monia. 

(c)  Combinations  of  hydrogen,  nitrogen  and  carbon 
into  aniline  and  many  other  compounds. 

(d)  Combinations   of   carbon,    hydrogen   and   oxygen 
into  phenol  and  other  compounds. 

(I)  Combinations  of  carbon  and  oxygen  into  carbon 
monoxide  and  carbon  dioxide. 

(/)   Pure  hydrogen. 

(g)  A  nearly  pure  residue  of  carbon  which  is  called 
coke. 

When 'sulphur  is  present,  sulphur  dioxide  and  other 
compounds  of  sulphur  and  the  other  elements  present  are 
produced. 

Charcoal.  —  Wood  may  also  be  subjected  to  destruc- 
tive distillation,  the  process  being  essentially  the  same  as 
that  just  described.  The  carbon  residue  is  called  charcoal. 


16  MATERIALS  OF  MACHINES 

The  object  of  the  processes  for  the  production  of  coke 
and  charcoal  is  to  produce  a  concentrated  fuel  by  removing 
all  substances  except  the  available  fuel  element,  carbon. 
Obviously,  this  increases  the  temperature  produced  by 
combustion. 

Gaseous  hydrocarbons,  carbon  monoxide  and  hydrogen 
are  gas  fuels  which  pass  off,  and  hence,  unless  these  are 
utilized,  the  process  sacrifices  a  portion  of  the  fuel  in  order 
to  increase  the  temperature  of  combustion. 

Pulverized  coal.  —  For  certain  service  pulverized 
coal  is  used  as  fuel  with  great  advantage.  Bituminous 
coal  is  dried  and  ground  so  that  about  90  per  cent  will 
pass  through  a  screen  of  100  meshes  to  the  inch;  this 
coal  powder  is  blown  into  a  furnace  in  a  cloud  where  it 
burns  while  in  suspension.  The  surface  of  contact  of  the 
coal  with  the  oxygen  of  the  air  is  vastly  increased  by 
pulverizing  and  it  is  unnecessary  to  supply  excess  of  air 
as  in  burning  coal  in  lumps;  in  fact,  the  air  supply  may  be 
kept  almost  at  the  theroetical  requirement  in  burning 
pulverized  coal,  with  the  result  that  the  temperature 
produced  approximates  the  temperature  limit  set  by 
dissociation  of  carbon  dioxide;  because' of  this,  it  has  been 
difficult  to  provide  refractory  lining  that  will  withstand 
the  temperature  of  combustion  of  powdered  coal.  This 
fuel  requires  a  very  large  combustion  chamber  as  the  coal 
powder  must  be  burned  while  in  suspension  and  the 
carbon  monoxide  formed  burns  with  a  very  long  flame;  for 
this  reason,  it  has  proved  successful  for  use  in  the  long 
rotary  kilns  used  in  the  manufacture  of  cement,  and  it 
has  failed  as  a  boiler  fuel,  though  it  might  possibly  be 
used  where  the  boiler  type  permits  large  combustion- 
space.  The' temperature,  also,  is  too  high  for  boiler  ser- 
vice, but  this  could  be  controlled  by  increase  in  the  air 
supply. 


PRELIMINARY   CONSIDERATION  OF  FUELS        17 

The  pulverized  coal  under  certain  conditions  forms 
an  explosive  mixture  with  the  oxygen  of  the  air,  and  some 
very  disastrous  explosions  have  resulted.  The  coal  now 
is  ground  only  as  it  is  needed;  it  is  never  stored  after 
grinding;  and  it  is  protected  from  the  air  while  in  transit 
to  the  furnace. 

Liquid  fuels.  —  The  liquid  fuels  of  greatest  impor- 
tance are  petroleum  and  alcohol. 

Crude  petroleum  consists  of  a  complex  combination 
of  hydrocarbons,  with  great  variations  according  to  its 
source,  together  with  small  and  varying  proportions  of 
oxygen,  nitrogen  and  sulphur.  The  constituent  hydro- 
carbons of  any  crude  petroleum  vary  in  composition 
from  C4Hio  through  a  long  series  of  proportions  to  Ci3H23 
with  steadily  reduced  proportion  of  hydrogen,  and  with 
increased  density  and  reduced  tendency  to  vaporize. 
If  the  temperature  of  crude  petroleum  is  steadily  raised, 
the  hydrocarbons  distill  off  in  an  order  determined  by  the 
proportion  of  hydrogen  present;  first  gasoline  of  various 
grades,  then  kerosene  of  various  grades,  and  then  lubri- 
cating oils  of  various  grades,  leaving  a  residue  of  hydro- 
carbons that  are  solid  at  ordinary  temperatures.  There 
may  be  also  a  residue  of  coke,  that  is,  carbon  uncombined 
with  hydrogen. 

Crude  petroleum  is  used  as  a  fuel  in  the  furnaces  of 
steam  boilers  and  in  several  types  of  metallurgical  fur- 
naces. Special  burners  are  used  and  provision  is  made 
for,  (a)  preheating  the  oil  to  increase  fluidity;  (6)  sup- 
plying the  oil  under  pressure;  (c)  delivering  the  oil  for 
combustion  in  a  very  fine  spray  by  the  use  of  steam  or 
compressed  air.  The  temperature  limit  is  the  same  as 
for  carbon  and  hydrogen  in  other  forms;  but  the  air  to 
support  combustion  and  the  oil  of  the  spray  can  be  so 
intimately  mixed  that  it  is  unnecessary  to  supply  an 


18  MATERIALS  OF  MACHINES 

excess  of  air  and  hence,  the  theoretical  limit  of  temperature 
can  be  approached  more  nearly  than  with  solid  fuel. 

Alcohol  is  not  yet  an  important  industrial  fuel,  and  it  is 
not  used  at  all  for  metallurgical  purposes;  but  it  may 
in  the  future  become  a  very  important  factor  in  metal- 
lurgy and  power  development. 

The  supplies  of  coal,  oil  and  natural  gas  will  eventually 
be  exhausted,  since  there  is  continual  draft  on  a  store 
that  is  never  renewed,  and  then  it  will  be  necessary  to 
supply  heat  energy  for  human  uses  from  the  sun's  energy 
stored  by  plant  growth  of  the  present  time.  As  already 
stated,  wherever  plants  grow,  carbon  is  taken  from  the 
carbon  dioxide  of  the  atmosphere  and  hydrogen  is  taken 
from  the  water  of  the  sap  and  they  are  stored  together 
with  oxygen  as  cellulose,  starch  or  sugar. 

Cellulose,  the  chief  constituent  of  plant  fiber,  and  starch, 
the  chief  constituent  of  grains,  potatoes,  etc.,  are  isomeric; 
that  is,  they  contain  the  same  elements  in  the  same  pro- 
portion, CeHioOs,  but,  probably  because  of  some  different 
arrangement  of  atoms,  they  are  very  different  substances. 
Sugar,  chemically  C^H^On,  is  found  in  sugar  cane,  sugar 
beets  and  other  products  of  plant  growth.  Sugar  or 
starch  mixed  with  water  and  fermented  with  yeast  yields 
a  weak  solution  of  alcohol  which  may  be  condensed  by 
distillation,  yielding  ethyl  alcohol,  C2H5OH.  In  starch, 
C6Hio05,  hydrogen  and  oxygen  are  present  in  proportions 
to  form  water  and  hence  only  the  carbon  is  available  for 
fuel,  while  the  water,  since  it  must  be  heated,  vaporized 
and  superheated,  would  absorb  a  part  of  the  heat  of  com- 
bustion, making  it  unavailable  and  thus  reducing  the 
temperature  of  combustion.  This  is  also  true  of  sugar 
and  hence  they  are  not  good  fuels.  But  the  alcohol  pro- 
duced, C2H5OH,  has  four  atoms  of  hydrogen  available 
for  every  two  atoms  of  carbon  and  only  one  corresponding 


PRELIMINARY  CONSIDERATION  OF  FUELS        19 

molecule  of  water  to  absorb  heat.  Henee,  the  process 
that  has  changed  starch  or  sugar  into  alcohol  has  greatly 
increased  the  fuel  value. 

When  cellulose,  C6Hi005,  is  subjected  to  dry  distilla- 
tion at  high  temperatures  it  yields  liquid  products  from 
which  methyl  alcohol,  or  wood  alcohol,  CH3OH,  may  be 
separated.  This  change  is  also  accompanied  by  an  in- 
crease in  fuel  value. 

If  lack  of  other  fuel  and  reduced  price  of  alcohol  made 
it  desirable,  alcohol  could  be  burned  effectively  by  the 
method  used  for  burning  crude  petroleum,  as  a  source 
of  heat  for  metallurgical  furnaces  or  for  steam  boilers. 
Alcohol  could  also  replace  the  more  volatile  petroleum 
products  in  internal  combustion  engines.  This  would 
utilize  energy  coming  from  the  sun  year  by  year  in  the 
present,  and  so  the  draft  would  be  on  a  store  that  nature 
continually  renews.  Hence,  it  offers  a  possible  solution 
of  the  fuel  problem  of  the  future. 

Gas  fuel  has  several  advantages  over  solid  fuel  for 
many  metallurgical  processes. 

1.  Inferior  solid  fuel  -may  be  used  for  the  generation 
of  the  gas  fuel. 

2.  The  furnace  for  the  production  of  the  gas  may  be 
at  a  distance  from  the  furnace  where  the  gas  is  used,  the 
transfer  being  made  through  pipes  with  resulting  saving 
of  valuable  space. 

3.  Heat  may  be  more  easily  applied  uniformly  over  a 
given  surface,  or  concentrated  locally,  with  gas  fuel  than 
with  solid  fuel. 

4.  The  air  which  supports  combustion  can  be  much 
more  completely  mixed  with  the  fuel,  and  therefore,  the 
excess  of  air  over  that  necessary  for  complete  combustion 
is  reduced  to  a  minimum,  with  a  resulting  increase  in 
temperature. 


20  MATERIALS  OF  MACHINES 

5.  If  the  mixture  of  air  and  gas  is  properly  regulated, 
there  will  be  a  complete  absence  of  smoke  and  soot,  and 
the  latter  will  not  be  mixed  with  the  material  treated. 

Gas  fuel  may  be  either  natural  or  artificial.  Natural 
gas,  like  coal  and  petroleum,  is  a  product  of  nature's 
processes  acting  through  long  periods  of  time  upon  prod- 
ucts of  plant  growth.  Natural  reservoirs  of  this  gas 
are  tapped  by  drilling,  usually  in  petroleum  regions,  and 
are  piped  to  places  where  the  gas  is  used.  Natural  gas 
consists  chiefly  of  marsh  gas,  CH4,  with  small  amounts 
of  other  hydrocarbons,  hydrogen  and  carbon  monoxide. 
All  of  these  constituents  are  combustible  and  hence,  the 
heat  value  is  high;  from  800  to  970  B.t.u.  per  cubic  foot 
of  gas  under  standard  conditions  of  pressure  and  tem- 
perature.* 

Natural  gas  is  available  only  in  a  few  limited  localities; 
and,  since  it  is  a  stored  product  of  nature's  very  slow 
processes,  the  reservoirs  are  eventually  exhausted.  While 
it  lasts  it  is  a  very  valuable  fuel. 

There  are  three  processes  for  the  production  of  arti- 
ficial gas  fuels : 

1.  Illuminating-gas  process.  —  This  process  consists  of 
the  destructive  distillation  of  coal  containing  a  large  per- 
centage of  volatile  matter.  It  is  similar  to  the  process 
for  production  of  coke,  with  the  difference  that  gas  is  now 
the  product  and  coke  the  by-product.  The  composition 
of  the  gas  varies  with  the  fuel  and  with  the  conditions  of 
operation,  but  the  variation  is  not  usually  very  great 
from  the  following  analysis: 

Hydrogen,  H 49  per  cent  by  volume. 

Marsh  gas,  CH4 34  per  cent  by  volume. 

Carbon  monoxide,  CO ...     8  per  cent  by  volume. 

Ethylene,  C2H4 4  per  cent  by  volume. 

Benzene,  CeHe 1  per  cent  by  volume. 

*  See  "Gas  Power"  by  Hirshfeld  and  Ulbricht,  page  15. 


PRELIMINARY  CONSIDERATION  OF  FUELS   21 

The  gas  also  contains  small  amounts  of  incombustible 
nitrogen,  carbon  dioxide  and  water  vapor. 

The  flame  from  this  gas  is  luminous  because  of  the 
presence  of  the  hydrocarbons,  C2H4  and  CeH6.  When 
these  burn  with  restricted  oxygen  supply,  carbon  is  sep- 
arated as  a  finely  divided  solid  which  becomes  incan- 
descent and  luminous  at  the  flame  temperature,  and 
which  burns  to  C02  on  reaching  the  flame  limit.  When 
the  oxygen  supply  is  adequate,  as  in  the  Bunsen  burner, 
combustion  is  complete,  no  solid  carbon  appears  and  the 
flame  is  not  luminous.  The  non-luminous  flame  may,  of 
course,  be  used  for  light  with  mantle  burners. 

2.  Water-gas  process.  —  In  this  process  steam  is  passed 
through  a  bed  of  incandescent  carbon.  The  reactions 
are  as  follows: 

C2  +  4  H2O  =  2  C02  +  4  H2. 

2CO2  +  C2  =4  CO. 

CO  +  H2O   =  C02  +  H2. 

These  reactions  probably  go  on  simultaneously,  and  when 
the  process  is  properly  regulated,  the  composition  of  the 
resulting  gas  is  usually  within  the  following  limits: 

Carbon  dioxide,  C02 2  to  15  per  cent  by  volume. 

Carbon  monoxide,  CO. . .  20  to  40  per  cent  by  volume. 

Hydrogen,  H 50  to  65  per  cent  by  volume. 

Marsh  gas,  CH4 4  to   8  per  cent  by  volume. 

There  is  also  present  in  some  cases  a  small  amount  of 
ethylene,  C2H4;  but  this  is  not  usually  enough  to  make 
•  the  flame  luminous  and  hence,  if  the  gas  is  to  be  used  for 
illumination,  it  is  passed  through  a  second  furnace  where 
it  takes  up  the  vaporized  hydrocarbons,  C2H4  and  CeH6. 
The  breaking  up  of  the  steam  into  its  constituent  hydrogen 
and  oxygen  absorbs  heat,  and  this  heat  is  just  equal  to 
that  given  out  when  the  hydrogen  of  the  gas  is  burned 


22  MATERIALS  OF  MACHINES 

again.  Hence,  there  is  no  gain  in  heat  from  the  hydrogen 
that  .comes  from  the  steam;  in  fact,  there  is  a  loss  per 
pound  of  steam  corresponding  to  the  difference  in  heat 
carried  by  a  pound  of  steam  as  it  comes  to  the  water-gas 
furnace,  and  the  corresponding  pound  of  steam  (super- 
heated) as  it  is  produced  in  the  furnace  where  the  gas  is 
burned.  Hence,  water  gas  in  burning  gives  a  little  less 
heat  than  would  result  from  direct  burning  of  the  coal 
used  in  the  water-gas  furnace;  but  the  process  produces 
a  fuel  of  high  combustion  temperature  having  the  advan- 
tages of  the  gaseous  form,  see  page  19. 

3.  Producer-gas  process.  --  This  process,  the  most  im- 
portant to  the  metallurgist,  consists  of  burning  coal  with 
incomplete  oxygen  supply.  There  are  many  forms  of 
gas-producers  with  great  variation  in  details;  the  prin- 
ciples of  operation,  however,  can  be  explained  by  reference 
to  the  form  shown  in  Fig.  1.  It  consists  of  a  chamber,  A, 
lined  with  fire-brick,  and  having  a  suitable  grate  at  the 
bottom.  Coal  is  introduced  through  a  hopper,  B,  so 
arranged  that  communication  with  the  air  need  not  be 
made  when  the  solid  fuel  is  put  in.  Air  is  admitted 
through  the  grate,  and  at  D  there  is  a  steam-blower  used 
to  force  combustion  and  to  introduce  steam.  The  cham- 
ber is  connected  with  the  gas-flue  by  the  passage  C. 
The  most  rapid  combustion  occurs  near  the  grate.  Air 
passes  through  the  grate  and  its  oxygen  combines  with 
the  incandescent  carbon,  forming  carbon  dioxide,  CO2; 
this  in  passing  up  comes  in  contact  with  more  incandes- 
cent carbon  where  the  air  supply  is  limited  and  taking 
up  more  carbon  becomes  carbon  monoxide  which  passes 
up  into  the  chamber.  In  the  upper  part  of  the  coal 
where  the  heat  is  less  intense,  the  volatile  constituents 
distill  off;  in  fact,  the  action  is  the  same  as  in  illuminat- 
ing-gas retorts  with  the  production  of  hydrogen,  hydro- 


PRELIMINARY  CONSIDERATION  OF  FUELS        23 

carbons,  carbon  monoxide,  etc.  This  leaves  coke  which 
descends  slowly  becoming  incandescent  and  uniting  with 
oxygen  and  carbon  dioxide  to  form  carbon  monoxide.  Also, 
steam  from  the  blower  passes  through  the  grates  with 
just  the  same  result  as  in  the  water-gas  process  producing 


FIG.  1. 

hydrogen  and  carbon  monoxide.  Since  this  steam  is  de- 
composed into  its  constituents  with  absorption  of  heat, 
it  follows  that  when  the  hydrogen  burns  again  it  can  only 
restore  a  part  of  the  heat  it  has  received,  and  hence  the 
introduction  of  steam  does  not  add  to  the  heat  evolved 
in  the  furnace;  in  fact,  it  decreases  it.  But  though  it  is 
not  a  source  of  heat,  it  gives  a  convenient  auxiliary  means 
of  temperature  control  and  also  tends  to  prevent  clink- 
ering. 


24 


MATERIALS  OF  MACHINES 


An  average  of  the  resulting  gases  from  this  process  is 
follows : 


as  follows : 

Combustible . . . 

Incombustible. . 


CO ...  24.  2  per  cent  by  volume. 

H 8.2  per  cent  by  volume. 

CH4 ....  2.2  per  cent  by  volume. 

f  CO2 ....   4.2  per  cent  by  volume. 

'IN 61. 2  per  cent  by  volume. 


Therefore,  34.6  per  cent  of  this  gas  is  combustible, 
while  65.4  per  cent  is  incombustible,  and  hence  its  com- 
bustion temperature  must  be  low.  It  would  seem, 
therefore,  that  " producer  gas"  could  not  be  used  for 
high  temperatures.  It  becomes  available  for  this  pur- 
pose, however,  through  the  regenerative  furnace,  orig- 
inally invented  by  Messrs.  Frederick  and  C.  W.  Siemens. 
The  gas,  instead  of  being  admitted  to  the  furnace  directly, 
passes  through  a  chamber,  B  (Fig.  2),  filled  with  "  chequer 


FIG.  2. 

work,"  i.e.,  full  of  small  intricate  passages,  surrounded  by 
refractory  material  suitable  for  storage  of  heat.  The  air 
also  passes  through  a  similar  chamber,  A,  and  meets  the  gas 
at  C,  the  entrance  to  the  hearth  Z),  where  the  metal  is 


PRELIMINARY  CONSIDERATION  OF  FUELS   25 

treated.  The  air  is  admitted  above  the  gas,  so  that,  because 
of  its  greater  specific  gravity,  it  shall  mix  more  completely 
with  the  gas.  Combustion  occurs  at  C,  and  the  products 
of  the  combustion,  heated  to  a  temperature  corresponding 
to  the  combustion  temperature  of  •  the  fuel,  pass  over  the 
hearth  where  they  fulfill  their  function  in  the  treatment 
of  the  charge  with  loss  of  heat  and  reduction  of  tem- 
perature, and  then  pass  on,  still  at  very  high  tempera- 
tures, through  the  chambers  A\  and  BI  to  the  stack.  In 
passing  they  heat  up  these  chambers  to  their  own  tem- 
perature, if  the  process  is  sufficiently  long  continued. 
Then  the  connections  are  changed  so  that  the  gas  comes  in 
through  BI,  and  the  air  supply  through  AI,  and  A  and  B 
are  connected  with  the  stack.  The  gas  and  air  passing 
through  the  heated  chambers  have  their  temperature 
raised  before  combustion  takes  place;  then  the  tempera- 
ture is  still  further  raised  by  the  combustion,  so  that  the 
products  of  combustion  now  pass  to  the  stack  after  use 
in  the  hearth  through  A  and  B  until  the  temperature  of 
these  chambers  is  raised  to  the  higher  temperature. 
Then  the  connections  are  again  reversed  and  the  entering 
gas  and  air  are  heated  to  this  higher  temperature  before 
combustion,  and  so  on.  It  would  seem  that  an  indefi- 
nitely high  temperature  could  be  produced  by  this  method, 
but  it  cannot  because  a  limit,  about  3500°  F.,  is  set  by 
dissociation  of  CO2  into  carbon  and  oxygen  at  high  tem- 
peratures with  absorption  of  heat.  See  page  10. 


CHAPTER  II 
ELECTRIC  FURNACES 

ELECTRIC  furnaces  are  considered  here  because  of  the 
bearing  their  use  has  on  the  temperatures  attainable  for 
industrial  purposes.  Other  types  of  furnaces  will  be 
explained  and  illustrated  in  connection  with  their  use  for 
metallurgical  purposes. 

Electric  furnaces  are  usually  classified  as;  arc,  resist- 
ance and  induction  furnaces. 

Arc  furnace.  —  If  carbon  electrodes,  which  have  been 
brought  into  contact  to  complete  an  electric  circuit  in 
which  a  suitable  current  is  maintained,  are  separated 
slightly,  the  carbon  of  the  separated  surfaces  and  the 
intervening  air  are  heated  by  the  passage  of  the  electricity 
across  the  gap.  As  a  result  of  this  heating,  the  opposing 
surfaces  of  the  electrodes  first  grow  red  and  then  white 
and  some  of  the  surface  carbon  is  vaporized ;  the  resulting 
carbon  vapor  fills  the  gap,  the  electricity  flows  through 
the  vapor,  and  the  gap  may  then  be  increased,  because 
the  resistance  of  the  carbon  vapor  is  less  than  the  resist- 
ance of  the  air.  The  electricity  thus  flowing  raises  the 
temperature  of  the  ends  of  the  electrodes  and  the  inter- 
vening carbon  vapor  to  incandescence  and  electrical 
energy  is  transformed  into  heat  and  light.  The  light 
may  be  used  for  illumination,  as  in  the  arc  lamp,  or  the 
heat  may  be  used  as  in  the  arc-type  electric  furnace.  A 
limit  is  set  to  the  temperature  that  can  be  produced  in 
this  furnace;  this  limit  depends  on  the  supply  and  dis- 
posal of  heat.  The  heat  supplied  by  transformation  of 
electrical  energy  in  the  arc  is  (a)  radiated  to  the  furnace, 

26 


ELECTRIC  FURNACES  27 

or,  (6)  applied  to  the  vaporization  of  carbon,  (a)  raises 
the  temperature  of  the  contents  of  the  furnace,  but  (b) 
disappears  as  sensible  heat  and,  therefore,  does  not  affect 
temperature.  As  the  temperature  rises  the  amount  of 
carbon  vaporized  increases  and,  therefore,  the  amount  of 
heat  abstracted  from  the  energy  supply  for  this  purpose 
increases  and  the  heat  left  over  to  raise  temperature 
grows  less.  A  temperature  would  finally  be  reached  at 
which  an  increase  in  heat  evolved  (by  the  increase  in 
electrical  energy  in  the  circuit)  would  be  met  by  an  equal 
disappearance  of  heat  to  maintain  increased  vaporiza- 
tion of  carbon,  and  with  these  conditions  the  temperature 
would  reach  a  maximum.  With  an  electric  arc  between 
carbon  electrodes,  the  temperature  probably  may  reach 
about  6000°  F.  and  hence  the  electric  furnace  furnishes 
a  much  higher  temperature  than  a  furnace  for  the  com- 
bustion of  carbonaceous  fuel.  The  vaporization  of  the 
substances  treated  in  the  furnace  might  also  affect  the 
maximum  temperature. 

Resistance  type.  —  In  this  type  of  electric  furnace 
electrodes  have  their  terminal  surfaces  separated  by  a 
considerable  distance  and  the  space  between  them  is 
filled  by  some  substance,  "the  resistor,"  that  offers  suit- 
able resistance  to  the  passage  of  the  electricity.  When 
the  electricity  flows  its  energy  is  changed  into  heat  and 
light  in  the  resistor,  and  the  heat  is  passed  on  to  a  sub- 
stance that  needs  to  be  heated  for  some  useful  purpose. 
The  substance  treated  may  itself  form  the  resistor,  wholly 
or  in  part.  The  temperature  attainable  in  this  furnace 
is  the  temperature  at  which  there  is  equality  between 
any  increase  in  the  heat  supply  due  to  increased  flow  of 
electricity  in  the  circuit,  and  the  corresponding  increase 
in  heat  absorption  by  vaporization  of  the  resistor  or  the 
substance  treated. 


28  MATERIALS  OF  MACHINES 

Induction  type.  —  The  induction  coil  consists  of  two 
windings  of  insulated  wire  about  a  suitable  soft  iron  core. 
When  alternating-current  electricity  flows  in  one  coil, 
called  the  primary,  an  alternating-current  is  induced  in  the 
closed  secondary  coil.  The  voltages  of  the  primary  and 
induced  currents  are  nearly  directly  proportional  to  the 
number  of  turns  of  wire  in  the  respective  coils.  Hence, 
if  the  primary  has  a  large  number  of  turns  and  the  sec- 
ondary a  small  number  of  turns,  a  high  voltage  current 
in  the  primary  would  induce  a  low  voltage  current  in 
the  secondary.  This  is,  of  course,  the  principle  of  the 
alternating-current  transformer.  Now,  if  the  material 
to  be  treated  in  the  furnace  can  replace  the  secondary 
coil  a  current  will  be  induced  in  it  by  the  current  in  the 
primary  coil,  and  if  this  current  is  suitable  the  required 
heating  effect  will  be  produced.  In  this  type,  if  the  sec- 
ondary coil  is  not  closed  a  current  in  the  primary  would 
induce  an  electromotive  force  in  the  secondary,  but 
electricity  would  not  flow.  In  some  cases,  the  material 
treated  is  a  solid  which  is  liquefied  by  heat  from  the  cur- 
rent. In  the  induction-type  furnace,  the  secondary  is 
closed  by  this  liquid  and  the  closing  cannot  be  effected 
until  the  solid  is  melted  and  the  solid  cannot  be  melted 
until  the  secondary  is  closed.  Hence,  in  starting  the 
furnace,  it  is  necessary  to  introduce  some  other  material 
until  working  conditions  are  established.  This  type  is, 
therefore,  better  fitted  for  continuous  than  for  interrupted 
service.  The  temperature  is  limited  in  this  furnace 
exactly  as  in  the  others. 

Combination  of  types.  —  In  the  arc-type  furnace  the 
electrodes  often  project  vertically  downward  into  the 
furnace  with  suitable  arcing  distance  between  their  ter- 
minal surfaces  and  the  material  treated;  two  arcs  are 
thus  formed  and  the  electricity  also  passes  through  the 


ELECTRIC  FURNACES  29 

material  treated,  which  thus  acts  as  a  resistor.  Hence, 
this  type  is  really  a  combination  of  arc  and  resistance 
furnaces.  The  arc  furnace  is  virtually  a  resistance  furnace 
in  which  the  vapor  between  the  electrodes  is  the  resistor. 
In  the  induction  furnace  heat  is  produced  by  resistance 
to  the  flow  of  electricity  in  material  treated  and  hence, 
this  type  really  uses  a  combination  of  the  induction  and 
resistance  principles. 


CHAPTER  III 
REFRACTORY  MATERIALS 

CRUCIBLES  and  the  linings  of  furnaces,  ladles  and  other 
apparatus  for  metallurgical  purposes  must  be  made  of 
materials  having  suitable  resistance  to  fusion,  to  change 
of  form  at  high  temperature  and  to  wasting  by  chemical 
or  erosive  action;  also  these  materials  must  be  so  consti- 
tuted as  not  to  interfere  with  desired  chemical  changes, 
or  to  cause  undesirable  chemical  changes,  in  the  materials 
treated  by  the  process. 

Acid,  neutral  or  basic  linings  for  furnaces.  —  Tem- 
peratures of  incipient  fusion  of  pure  refractories  are 
approximately  as  follows: 

Silica,  Si02  (acid) 3200°  F. 

Aluminum  silicate,  Al2Si2O7  (neutral) .  .   3300°  F. 

Chromic  oxide,  Cr203  (neutral) infusible  * 

Carbon,  coke  or  graphite  (neutral) ....   infusible 

Alumina,  A1203  (basic) 3600°  F. 

Lime,  CaO  (basic) 4500°  F. 

Magnesia,  MgO  (basic) 4500°  F. 

This  table  shows  silica  to  be  the  least  satisfactory 
material  for  use  as  a  refractory,  considering  only  the  tem- 
perature of  fusion.  But,  because  of  other  qualities,  it  is 
used  very  extensively  where  the  temperature  to  be  sus- 
tained is  safely  below  fusion  point. 

Lime,  CaO,  in  contact  with  aluminum  silicate  at  high 
temperatures  yields  calcium  silicate  and  calcic  aluminate, 

*  This  means  infusible  at  the  maximum  temperatures  now  used 
in  industrial  processes. 

30 


REFRACTORY   MATERIALS  31 

and  the  mixture  is  fusible  at  a  relatively  low  temperature, 
probably  about  2000°  F.  If  magnesia  is  substituted  for 
lime  there  is  a  similar  reduction  of  fusion  temperature. 
Hence,  lime  or  magnesia  would  act  as  a  flux  upon  alumi- 
num silicate;  and,  conversely,  aluminum  silicate  would 
act  as  a  flux  upon  lime  or  magnesia.  Hence,  wherever 
alumina,  silica  and  lime  are  in  contact,  or  wherever 
alumina,  silica  and  magnesia  are  in  contact,  if  the  temper- 
ature is  above  2000°  F.  fluxing  will  take  place;  that  is, 
the  material  will  melt.  In  certain  cases  this  melting  is 
desirable,  as,  for  instance,  when  refractory  alumina  and 
silica  are  removed  from  the  blast-furnace  as  a  fluid  slag 
by  the  fluxing  agency  of  lime  that  is  introduced  for  this 
purpose  (see  page  42).  But  furnace  linings  must  not 
be  fluxed  away,  and  hence  the  coming  together  of  alumina, 
silica  and  lime,  or  of  alumina,  silica  and  magnesia  should 
be  prevented  where  temperatures  exceed  2000°  F.r  if  any 
one  of  the  substances  is  an  essential  part  of  the  furnace 
lining.  Ferrous  oxide,  FeO,  also  acts  as  a  flux  upon 
aluminum  silicate. 

If  a  metallurgical  process  that  produces  a  slag  contain- 
ing lime  or  magnesia  or  ferrous  oxide  is  carried  on  in  a 
furnace  lined  with  material  containing  excess  of  silica, 
that  is,  with  an  acid  lining,  the  lining  will  be  fluxed 
away.  Also,  with  a  basic  lining  and  an  acid  slag,  the 
lining  will  be  fluxed  away.  Hence,  a  process  producing 
an  acid  slag  should  be  carried  on  in  a  furnace  having  an 
acid  lining,  and  a  process  producing  a  basic  slag  should 
be  carried  on  in  a  furnace  having  a  basic  lining. 

A  neutral  lining  would  be  best,  because  it  would  resist 
the  fluxing  action  of  either  an  acid  or  a  basic  slag;  but 
with  the  neutral  materials  at  present  available,  it  is 
difficult  to  prevent  destruction  of  the  linings  by  other 
causes. 


32  MATERIALS  OF  MACHINES 

Kaolin  is  a  fine  white  clay  used  as  an  ingredient  of 
porcelain  and  other  white  ware.  It  consists  of  a  mixture 
of  hydrated  aluminum  silicate  with  other  substances  like 
hydrous  aluminum  oxides,  feldspar,  quartz  and  mica. 
It  has  the  quality  of  becoming  plastic  when  mixed  with 
water,  and  it  thus  can  be  molded  into  required  forms 
which  may  be  dried  at  low  or  moderate  temperatures  and 
calcined  at  higher  temperatures  with  the  removal  of  the 
water  of  hydration  and  with  partial  fusion.  There 
results  a  hard,  strong,  refractory  material. 

Fire-clay  is  a  clay  that  is  capable  of  withstanding 
high  temperatures,  say  a  minimum  of  3000°  F.  Fire- 
clay in  addition  to  hydrated  aluminum  silicate  usually 
contains  varying  amounts  of  lime,  magnesia  and  iron 
oxide;  these,  through  their  fluxing  action,  increase  the 
fusibility  of  the  calcined  material;  and  hence,  the  higher 
the  temperature  to  be  sustained  the  greater  the  need 
that  these  substances  should  be  reduced  to  a  minimum. 
Fire-clays  vary  greatly  in  composition  and  in  physical 
properties,  and  hence  must  be  chosen  with  great  care 
according  to  the  service  required;  whether  it  is  resistance 
to  fluxing  or  abrasion,  or  to  the  action  of  gases,  or  capac- 
ity for  enduring  temperature  changes  safely.  During 
the  calcining  of  fire-clay  considerable  shrinkage  occurs, 
which  may  be  accompanied  by  distortion  and  cracking. 
This  shrinkage  may  be  reduced  by  mixing  coke  dust, 
graphite  or  silica  sand,  or  more  commonly  ground  fire- 
brick or  flint  clay  with  the  plastic  clay.  The  fitness  of 
clays  for  use  in  refractories  for  given  service  depends  not 
only  upon  the  chemical  composition,  but  also  upon  the 
physical  condition  and  manner  of  burning.  Thus  density, 
porosity  and  condition  resulting  from  variation  of  tem- 
perature and  duration  of  the  burning  affect  refractori- 
ness. 


REFRACTORY   MATERIALS  33 

Silica,  when  pure,  cannot  be  made  into  fire-brick  or 
used  in  mass  for  furnace  linings,  because  a  binding  material 
is  necessary.  Silica  rock,  however,  often  contains  enough 
clay  to  serve  for  binding;  and  often  a  mixture  of  ground 
quartzite  or  silica  sand  with  fire-clay  is  used. 

Silica  bricks  are  also  made  by  mixing  fine  silica  with 
a  very  small  proportion  of  lime  and  adding  water  until  the 
mass  is  somewhat  coherent,  when  it  is  molded  under 
high  pressure,  dried  and  fired.  During  the  firing  the  lime 
combines  with  a  small  part  of  the  silica  and  with  such 
small  amounts  of  aluminum  as  may  be  present  to  form  a 
fusible  slag  that  acts  as  a  binder  for  the  silica. 

A  natural  or  artificial  mixture  of  a  large  proportion  of 
siliceous  material  —  usually  not  less  than  90  per  cent  — 
with  clay  or  lime  or  both  is  usually  called  gainster. 

Chromite,  FeCr2O4,  a  double  oxide  of  iron  and  chro- 
mium, is  neutral  and  very  infusible.  It  occurs  in  nature 
as  "chrome  ore,"  which  contains  also  alumina,  magnesia, 
lime  and  silica.  Proportions  vary,  but  are  often  about 

as  follows : 

Per  cent 

Chromic  oxide 50 

Ferrous  oxide 35 

Alumina 3 

Magnesia 4 

Lime 5 

Silica 2 

This  being  neutral  would  be  almost  an  ideal  material 
for  furnace  bottoms  if  it  were  not  for  the  fact  that,  after 
it  is  molded  into  place,  it  is  almost  impossible  to  produce  a 
temperature  high  enough  to  cause  it  to  set  thoroughly, 
and  this  leaves  it  liable  to  destruction  by  mechanical 
erosion.  The  fusibility  of  the  material  varies  with  the 
amount  and  proportion  of  substances  other  than  chromic 


34  MATERIALS  OF   MACHINES 

oxide  present,  and  when  properly  selected,  it  is  a  very 
valuable  material  for  daubing  and  patching  furnace 
linings. 

Carbon  in  the  form  of  coke  or  graphite  is  used  as  a 
refractory  material  either  in  bricks  or  crucibles.  But 
though  carbon  is  neutral  and  infusible,  it  is  such  a  strong 
reducing  agent,  that  is,  it  has  such  a  strong  tendency  to 
unite  chemically  with  oxygen,  that  in  the  presence  of 
oxygen  or  oxides  of  other  substances  carbon  monoxide  or 
carbon  dioxide  is  formed  and  passes  off  as  gas,  thus 
eventually  destroying  the  crucible  or  the  furnace  lining. 
Carbon,  therefore,  can  only  be  used  as  a  refractory  where 
oxygen  is  excluded. 

In  making  carbon  refractory  forms,  tar  is  sometimes 
used  as  a  binding  material  for  the  coke  dust  or  ground 
graphite.  The  mixture  is  made,  formed  and  dried  and 
then  fired  with  exclusion  of  oxygen.  The  tar  is  coked 
and  the  product  is  practically  pure  carbon. 

Clay  is  also  used  as  binding  material  for  finely  divided 
carbon.  A  mixture  is  made  of  coke  or  graphite,  clay  and 
water;  the  mixture  is  formed,  dried  and  fired.  Crucibles 
made  by  these  methods  are  used  for  melting  steel  in  the 
crucible  process. 

Bauxite  is  a  mixture  of  a  large  proportion  of  hydrated 
alumina  A1203  •  2H20,  with  clay,  silica,  iron  oxide  and  titanic 
oxide ;  another  hydrated  aluminum  oxide,  A1203  •  3  H20,  is 
also  often  present.  Bauxite  is  used  for  making  basic  re- 
fractory bricks  for  furnace  linings.  To  be  suitable  for 
this  purpose  it  should  contain,  after  calcining,  about  90 
per  cent  of  alumina.  There  is  a  large  amount  of  combined 
water  in  the  raw  bauxite,  a  probable  maximum  being  30 
per  cent,  and  the  removal  of  this  water  during  calcining 
causes  great  shrinkage  and  molds  must,  therefore,  be  made 
larger  than  the  required  size  of  the  finished  bricks.  These 


REFRACTORY  MATERIALS  35 

bricks  are  expensive  and  it  is  difficult  to  attain  a  tempera- 
ture in  firing  them  high  enough  to  produce  a  bond  that 
insures  requisite  strength.  Except  for  these  two  objec- 
tions, bauxite  bricks  provide  a  very  satisfactory  basic 
refractory. 

Lime,  CaO,  is  practically  infusible,  but  when  pure  is 
not  satisfactory  as  a  refractory  because  of  difficulty  in 
getting  it  to  bind,  and  also,  because  it  slakes  rapidly  on 
exposure  to  the  air  at  ordinary  temperatures,  forming 
calcium  hydroxide,  CaH2O2,  which  crumbles  and  wastes 
on  reheating.  Lime  refractories,  therefore,  cannot  be 
made  up  and  held  in  stock. 

Magnesia,  MgO,  obtained  by  calcining  magnesium 
carbonate,  MgCOs,  is  probably  the  best  material  for  basic 
furnace  linings.  Commercially  this  refractory  is  obtained 
from  "magnesite,"  a  mineral  found  in  Styria  and  Greece. 
The  calcined  magnesite  for  "  magnesite  bricks  "  contains 
from  80  to  90  per  cent  of  magnesia;  ferric  oxide,  alumina, 
lime  and  silica  are  present  in  varying  proportions.  The 
plasticity  of  calcined  magnesite  depends  on  the  temper- 
ature of  calcining;  if  this  temperature  is  about  1500°  F. 
the  product  has  a  specific  gravity  of  about  3,  while  higher 
calcining  temperatures  give  a  specific  gravity  of  about  3.7. 
The  former  is  plastic  enough  to  mold  under  pressure, 
while  the  latter  lacks  plasticity.  Magnesite  bricks  may 
be  made  of  a  mixture  of  from  four  to  six  parts  of  the 
heavier  with  one  part  of  the  lighter  calcined  magnesite 
with  from  ten  to  fifteen  per  cent  of  water.  This  mixture 
is  then  pressed  into  molds. 

The  only  objection  to  magnesia  for  basic  refractories 
is  that  it  is  expensive,  because  magnesite  is  found  only  in 
few  localities  far  away  from  metallurgical  centers.  For 
this  reason  "dolomite  "  or  magnesian  limestone  is  often 
used.  This  cheaper  mineral,  after  calcining,  contains  from 


36  MATERIALS  OF   MACHINES 

50  to  60  per  cent  of  lime,  30  to  40  per  cent  of  magnesia 
and  small  amounts  of  silica,  alumina  and  ferric  oxide. 
Calcined  dolomite  is  ground  fine  and,  either  with  or  without 
admixture  of  clay,  is  made  plastic  with  water,  formed  and 
fired.  Another  method  uses  tar  as  a  binding  material 
for  crushed,  calcined  dolomite.  Plastic  dolomite  is  ex- 
tensively used  for  daubing  and  patching. 


CHAPTER  IV 

OUTLINE  OF  THE  METALLURGY  OF  IRON 
AND  STEEL 

IRON  occurs  in  nature  combined  with  many  other  sub- 
stances. The  world's  supply  of  iron,  however,  is  ob- 
tained almost  exclusively  from  the  oxides  Fe203  and 
Fe304.  Carbonate  ores,  FeC03,  are  reduced  before 
smelting  by  roasting  to  FeO;  this  FeO  takes  up  more 
oxygen  from  the  atmosphere,  becoming  Fe20s.  The 
formation  of  these  iron  oxides  by  the  processes  of  nature 
was  accompanied  by  evolution  of  heat  energy.  This 
energy  per  unit  weight  of  oxide  was  definite  in  amount 
and  independent  of  the  method  or  time  of  formation. 
To  separate  the  oxide  again  into  its  constituents  an  exactly 
equivalent  amount  of  energy  must  be  supplied.  In  brief, 
the  separation  is  accomplished  as  follows:  Heat  energy 
is  supplied  to  the  iron  oxide  whereby  its  temperature  is 
raised.  The  bond  which  holds  the  iron  and  oxygen 
together,  whatever  its  character,  is  weakened.  But  this 
alone  is  insufficient  in  this  case  to  cause  separation. 
Therefore,  the  heating  is  caused  to  occur  in  the  presence 
of  carbon  (solid)  or  carbon  monoxide  (gas).  Either  of 
these  substances  has  greater  affinity  for  oxygen  at  high 
temperature  than  the  iron;  hence,  with  the  help  of  the 
heat,  is  able  to  pull  away  the  oxygen  from  the  iron  oxide, 
forming  C02,  which,  being  gaseous,  passes  off,  leaving 
the  iron.  The  heat  energy  that  is  effective  to  weaken 
the  bond  plus  the  energy  expended  by  the  carbon  or 
carbon  monoxide  in  pulling  away  the  oxygen  from  the  iron 

37 


38  MATERIALS  OF  MACHINES 

is  exactly  equal  to  the  heat  energy  that  was  evolved  by 
the  original  combination  of  the  oxygen  and  iron  into  iron 
oxide.*  The  real  process  is  much  more  complex  because 
of  circumstances  now  to  be  considered. 

Sources  of  iron.  —  Full  consideration  of  the  ores  of 
iron  is  beyond  the  scope  of  this  work.f 

Iron  ores  may  be  classified  as  follows: 

1.  Magnetic  oxide,  or  magnetite,  Fe3O4. 

2.  Ferric  oxide,  or  red  hematite,  Fe2O3. 

3.  Hydrated  ferric  oxide,  or  brown  hematite,  limonite, 
bog  ores,  etc. 

4.  Ferrous  carbonate  or  spathic  ore,  FeC03. 

These  ores  always  carry  other  substances,  and  the  pro- 
portions vary  between  wide  limits.  Sulphur  and  arsenic 
are  often  present,  and  these,  with  carbon  dioxide  and 
water,  may  be  removed  as  vapor  or  gas  at  comparatively 
low  temperatures  by  the  process  of  calcining  or  roasting. 

For  calcining  or  roasting  the  ore  is  piled  in  heaps  out 
of  doors,  or  charged  into  kilns,  with  fuel  in  proper  amount 
mixed  with  it.  The  fuel  is  ignited,  and  the  mass  slowly 
heated.  Water  is  driven  off  as  steam.  If  the  ore  is  car- 
bonate, FeCO3,  the  CO2  is  driven  off,  and  the  resulting 
FeO  is  changed  to  Fe203  by  combination  with  oxygen  of 
the  air.  If  any  iron  pyrites,  FeS2,  is  present  the  sulphur 
is  oxidized,  passing  off  as  SO2,  while  the  iron  is  also  oxi- 
dized, remaining  as  Fe2O3.  Arsenic  is  oxidized  and  vapor- 
ized if  present.  By  the  process  of  roasting,  the  structure 
of  the  ore  is  made  more  open,  and  hence  better  fitted  for 
smelting. 

*  By  the  law  of  conservation  of  energy. 

t  "Ore  Deposits  of  the  United  States,"  J.  F.  Kemp;  Scientific 
Publishing  Company.  "Iron  (the  Metallurgy  of),"  T.  Turner;  J.  B. 
Lippincott  Company. 


METALLURGY  OF  IRON  AND  STEEL  39 

When  roasting  is  carried  on  in  the  kilns  it  is  often  a 
continuous  process.  The  kiln  is  like  a  foundry  cupola, 
much  enlarged  in  diameter.  The  ore  and  fuel  are  charged 
in  at  the  top,  and  the  roasted  ore  is  withdrawn  from 
openings  at  the  bottom. 

The  process  is  now  usually  omitted  for  oxide  ores,  the 
roasting  being  accomplished  in  the  top  of  the  blast-furnace. 

The  early  methods  for  the  production  of  iron  were 
direct  methods,  i.e.,  the  product  was  wrought  iron,  which 
had  not  passed  through  the  intermediate  state  of  cast  iron. 

Chemically  these  methods  were  as  follows:  rich  ore, 
Fe203  or  Fe304,  was  charged  with  charcoal  into  a  rec- 
tangular hearth,  and  air-blast  was  supplied.  The  coal 
was  ignited  and  the  oxygen  of  the  air  combined  with  the 
carbon  of  the  fuel  to  form  CO2,  which  passing  on  over 
more  incandescent  carbon  was  reduced  to  CO,  which 
came  in  contact  with  the  Fe203,  when  the  following  reac- 
tions took  place: 

3  Fe203  +  CO  =  2  Fe3O4  +  CO2. 

2  Fe304  +  2  CO  =  6  FeO  +  2  C02. 

6  FeO  +  6  CO  =  3  Fe2  +  6  C02. 

Metallic  iron  and  C02  were,  therefore,  produced.  The 
silica  and  alumina  of  the  ore  united  with  FeO  formed  in 
the  process  —  see  reactions  above  —  to  form  a  double 
alumino-ferrous  silicate  or  slag,  which  is  fusible  at  a  low 
temperature,  and  which  was  partly  drawn  off,  while  the 
iron  remained  in  the  hearth  a  spongy  mass  filled  with 
molten  slag.  The  mass  was  then  heated  to  a  welding 
temperature  and  taken  to  a  hammer  or  squeezer,  where  the 
slag  was  removed  by  impact  or  pressure,  and  the  mass 
was  welded  into  a  bloom. 

The  details  of  this  process  varied  in  different  localities. 
Rich  ore  and  charcoal  for  fuel  were  required,  and  there 
was  great  waste  of  iron  in  the  slag.  It  was,  therefore,  a 


40  MATERIALS  OF   MACHINES 

very  expensive  process,  and  was  not  available  for  the  pro- 
duction of  large  quantities  of  iron. 

Nearly  all  the  iron  used  to-day  is  reduced  from  ore 
in  the  blast-furnace.  Fig.  3  shows  a  vertical  section  of 
a  blast-furnace.  The  height  varies  from  40  to  100  feet, 
and  the  diameter  at  M  varies  from  12  to  25  feet.  The 
inside  form  varies  with  the  kind  of  ore  and  fuel  used,  and 
with  the  pressure  and  quantity  of  air  of  the  blast. 

A  blowing  engine  supplies  air,  at  a  pressure  of  from  5  to 
15  pounds  per  square  inch,  to  the  large  pipe,  P,  which 
surrounds  the  stack.  At  intervals  of  the  circumference 
of  this  pipe  smaller  pipes  convey  the  air  to  the  tuyeres, 
T,  T,  which  deliver  it  into  the  furnace.  The  oxygen  of 
the  air  combines  with  carbon  of  the  fuel  and  forms  carbon 
dioxide,  which  is  almost  immediately  reduced,  in  the 
presence  of  carbon  with  restricted  oxygen  supply,  to 
carbon  monoxide.  There  is  a  constantly  ascending  cur- 
rent of  carbon  monoxide  and  nitrogen  through  the  con- 
stantly descending  solid  materials. 

The  "bell,"  B,  prevents  the  escape  of  gas  from  the  top 
of  the  stack,  and  insures  its  delivery  into  the  pipe,  G, 
Solid  materials  to  be  introduced  into  the  furnace  are 
placed  in  the  annular  space  above  B,  and  the  latter, 
which  is  controlled  by  power,  is  lowered  periodically  and 
the  charge  drops  into  the  furnace. 

The  function  of  the  blast-furnace  is  to  change  iron  ore 
into  pig  iron. 

Pig  iron  is  iron  carrying  from  3  to  10  per  cent  of  carbon, 
silicon,  manganese,  sulphur  and  phosphorus,  either  chem- 
ically combined  or  mechanically  mixed. 

The  blast-furnace,  therefore,  provides  for: 

(a)  The  removal  of  volatile  constituents  of  the  ore. 

(b)  The  reduction  of  the  iron  oxide  of  the  ore. 

(c)  The  removal  of  the  solid  earthy  constituents  of 
the  charge. 


METALLURGY   OF  IRON   AND  STEEL  41 


42  MATERIALS  OF  MACHINES 

It  also  provides  carbon,  silicon,  manganese,  sulphur  and 
phosphorus  under  proper  conditions  for  absorption  by 
the  iron. 

In  order  that  the  earthy  solids  of  the  ore  shall  combine 
with  the  flux  into  a  readily  fusible  slag,  silica,  alumina 
and  lime  must  be  present  (see  page  31).  If  the  ore 
carries  silica  and  alumina,  lime  will  act  as  a  flux,  and  it  is 
supplied  in  the  form  of  limestone.  If  an  ore  contains 
silica  only,  alumina  may  be  introduced  with  lime;  or, 
siliceous  and  aluminous  ores  may  be  mixed  and  fluxed 
with  limestone. 

Chemical  changes  in  the  blast-furnace.  —  Ore,  coke 
and  limestone  are  charged  into  the  top  of  the  stack  and 
descend  slowly  to  the  crucible,  0,  at  the  bottom,  with 
steadily  increasing  temperature.  The  ore  is  first  roasted, 
and  when  the  temperature  has  reached  about  450°  F.,  the 
reduction  of  iron  oxide  by  carbon  monoxide  begins  slowly 
and  continues  at  an  increasing  rate  until  the  temperature 
reaches  about  1100°  F.,  when  the  reduction  is  probably 
nearly  complete.  The  ore  has  now  become  a  sponge  of 
metallic  iron  mixed  with  silica,  alumina,  etc.  But  at  this 
latter  temperature  the  flux,  which  is  limestone,  CaCO3,  be- 
gins to  give  off  C02,  and  the  lime,  CaO,  thus  produced 
comes  in  contact  with  the  silica  and  alumina  of  the  re- 
duced ore,  and  they  descend  together  until  a  temperature 
is  reached  at  which  they  combine  to  form  a  fusible  slag, 
which  melts  and  leaves  the  iron  sponge. 

Introduction  of  carbon.  —  In  the  meantime  a  deposi- 
tion of  carbon  upon  the  iron  sponge  has  been  going  on. 
This  may  be  explained  as  follows:  when  carbon  mon- 
oxide passes  over  metallic  iron  at  a  temperature  of  about 
750°  F.,  the  carbon  monoxide  is  decomposed,  solid  carbon 
is  deposited,  and  carbon  dioxide  and  ferrous  oxide  are 
formed.  This  is  what  occurs  in  the  blast-furnace  when 


METALLURGY  OF  IRON  AND  STEEL  43 

the  temperature  of  about  750°  F.  is  reached  by  the  metallic 
iron  sponge.  Then,  as  the  temperature  rises,  ferrous 
oxide  thus  formed  is  reduced  again  by  carbon  monoxide, 
or  by  solid  carbon;  the  carbon  dioxide  passes  on  upward 
with  the  gas-current,  and  the  iron  sponge  remains  im- 
pregnated with  carbon.  As  this  passes  down  with  in- 
creasing temperature,  iron  carbide  is  formed,  which  is 
fusible  at  a  much  lower  temperature  than  pure  iron;  a 
temperature  which  is  reached  below  M  in  the  blast- 
furnace. Therefore,  the  descending  iron  carbide  is  raised 
to  its  fusion  temperature  and  melts  and  falls  into  the 
crucible  0. 

Introduction  of  silicon.  —  At  very  high  temperatures, 
in  the  lower  part  of  the  furnace,  where  carbon  and  silica 
and  metallic  iron  are  in  contact,  a  portion  of  the  silica  is 
reduced,  and  the  resulting  silicon  is  taken  up  by  the  iron; 
the  remaining  silica  goes  out  with  the  slag.  This  change 
is  favored  by  (a)  high  temperature,  (6)  excess  of  silica  in 
the  charge  and  (c)  deficiency  of  lime  in  the  slag.  Usu- 
ally not  more  than  5  per  cent  of  the  silica  of  the  charge 
is  reduced. 

Introduction  of  manganese.  —  The  manganous  oxide 
of  the  ore  is  not  reduced  by  carbon  monoxide,  but  is  in 
part  reduced  by  carbon  at  high  temperatures,  and  the 
resulting  manganese  combines  with  the  iron.  The  un- 
reduced manganous  oxide  passes  into  the  slag.  Certain 
ores,  like  New  Jersey  franklinite,  contain  very  large 
proportions  of  manganous  oxide,  and  the  product  of  their 
smelting  is  spiegeleisen,  or  ferro-manganese,  containing 
from  5  to  25  per  cent  manganese. 

Introduction  of  sulphur.  —  Only  a  small  part  of  the 
sulphur  in  the  charge,  which  is  chiefly  in  the  coke  as  iron 
sulphide,  FeS,  appears  in  the  pig  iron,  the  rest  passing 
into  the  slag  as  calcium  sulphide.  The  amount  of  sul- 


44  MATERIALS  OF   MACHINES 

phur  that  the  iron  can  take  up  depends  upon  the  capacity 
of  the  iron  itself  for  sulphur,  and  also  upon  the  amounts 
of  silicon  and  manganese  present  (see  page  128).  High 
furnace  temperature  tends  to  reduce  the  amount  of  sul- 
phur in  the  pig  iron.  Also  basic  slag,  i.e.,  slag  with  excess 
of  lime,  combines  readily  with  sulphur,  thereby  reducing 
the  amount  absorbed  by  the  iron. 

Introduction  of  phosphorus.  —  The  phosphorus  of 
the  charge  is  usually  in  the  ore  in  the  form  of  calcium 
phosphate.  This  is  not  changed  by  carbon  monoxide. 
But  when  the  part  of  the  furnace  is  reached  where  the 
slag  is  formed,  the  calcium  phosphate  is  reduced  in  the 
presence  of  solid  carbon.  The  lime  goes  to  the  slag; 
the  phosphoric  anhydride  is  broken  up  with  formation  of 
carbon  monoxide  and  iron  phosphide.  Practically  all 
of  the  phosphorus  of  the  charge  appears  in  the  pig  iron. 

Descent  of  coke.  —  Coke  is  the  fuel  almost  univer- 
sally used  in  "  hot-blast  "  furnaces.  As  the  coke  descends 
it  is  dried  and  raised  in  temperature.  It  meets  carbon 
dioxide,  which  may  come  from  the  reduction  of  iron 
oxide,  or  from  the  roasting  of  carbonate  ore,  or  from  the 
roasting  of  limestone.  The  carbon  dioxide  is  reduced  to 
carbon  monoxide  with  absorption  of  heat.  The  result- 
ing carbon  monoxide  may  take  part  again  in  reduction, 
or,  if  it  is  near  the  top  of  the  stack,  may  pass  off  unchanged 
with  the  gas-current.  The  coke  may  also  supply  part 
of  the  carbon  for  formation  of  carbureted  iron,  and  it 
also  helps  in  the  reduction  of  silica  and  phosphoric  an- 
hydride. When  it  reaches  the  vicinity  of  the  tuyeres 
it  burns  to  CO  and  evolves  the  heat  necessary  for  the 
operation  of  the  furnace.  All  of  the  carbon  of  the  coke 
appears  either  in  the  pig  iron  or  in  the  gases  issuing  from 
the  top  of  the  stack. 

The  combination  of  iron,  carbon,  silicon,  manganese, 


METALLURGY  OF  IRON  AND  STEEL  45 

sulphur  and  phosphorus  is  fusible  at  a  temperature  which 
is  reached  a  little  below  M  in  the  blast-furnace.  Hence, 
fusion  occurs  and  the  melted  substance  falls  into  the 
crucible,  0,  together  with  the  fluid  slag.  The  iron  and 
slag  separate  because  of  their  difference  in  specific  grav- 
ity, the  slag  floating  on  the  top.  When  a  sufficient 
amount  has  accumulated,  the  slag  is  tapped  out  through 
the  "  cinder-notch,"  allowed  to  cool,  and  transferred  to 
the  "  cinder  dump."  The  iron  is  tapped  out  through  a  hole 
low  down  in  the  crucible  and  allowed  to  run  out  through 
properly  formed  sand  channels,  where  it  cools  as  pig  iron. 

Fig.  4  gives  Professor  Diederich's  diagrammatic  sum- 
mary of  blast-furnace  operation.  While  this  does  not 
pretend  to  give  exhaustively  all  changes  that  occur,  it 
does  give  the  changes  that  are  fundamentally  important. 
The  figure  is  self-explanatory. 

Obviously,  a  continuous  current  of  gas  flows  from  the 
top  of  the  blast-furnace  stack.  The  chief  constituents 
of  this  are  nitrogen,  carbon  dioxide  and  carbon  monoxide. 
When  the  furnace  is  properly  regulated,  the  carbon  mon- 
oxide equals  about  25  per  cent  of  the  issuing  gas.  It  is, 
therefore,  a  gas  fuel.  A  portion  of  this  is  often  burned  in 
the  boiler  furnaces  to  make  steam  to  run  the  blowing 
engines  and  other  auxiliary  machinery.  In  some  modern 
blast-furnace  plants  the  stack  gas  is  used  as  a  fuel  in  in- 
ternal-combustion engines  which  develop  power  for  the 
entire  plant.  The  rest  of  the  stack  gas  is  used  for  heating 
the  blast. 

In  early  blast-furnaces  the  blast  entered  the  furnace 
nearly  at  the  temperature  of  the  outside  air.  Cold  blast 
is  still  used  in  furnaces  for  the  smelting  of  some  special 
grades  of  iron. 

Heating  the  blast  on  its  way  from  the  blowing  engine 
to  the  tuyeres  results  in: 


46 


MATERIALS  OF  MACHINES 


8       8 


8 

+ 

r° 

P=H 

II 

O 

+ 
9 


1-8 


PS11I 
a  I  a  i-s 

•sls^i 
^s  * 

ff£ 

0.   >5-^"^     O 

=l^g^^ 

03 ^3  S-^^3 
SS^Ig 

hill 

3  s*5  g 

fl-^o  >£ 

MSe-§8 


og§§^ 

^|ggs 
.asl^jS 

_r!"S«S3i 

^  iw 

l-S'HIg 

o  <3  o  ^  w 

5-3^0. 


ir. 


j 

C3   0 .13  "  b 
^  >y2 


^^.S 


J 


I! 


e^ 
8| 


•a -a 

^ 

oT^ 
>  a 


%£ 


i  B  i  m 

S^M       W4-7  + 


o 
gl 

all-d 

*fe°l 

S^JS 

igHg 

s      o 

QQ     O 

iS"°  08 
^,33  a 

^'§& 

£  S  S 


"+  +  + 

±a£™ 

£S  <NU 
II    II    II    II 

oooo 

<M  c<i  *o  _i_ 

+  +  +0 


90Q 

«2£fC 


ouoz  uoiJdoD.i9!)ni  ^BOH 


3uoz  uoisnj 
ao  Samaras 


I 

»     |_|_5l      1    .  : 

f                                   ^i 



, 

5         ,              ,              , 

r                     •> 

^       1 

^T 

i 

u  

^      i     ^s  j 
i 

^-"""T 

•43 


I 


METALLURGY  OF  IRON  AND  STEEL  47 

Higher  temperature  (see  page  11). 

Economy  of  fuel,  because  the  blast  is  heated  by  the 
waste  gas  fuel  from  the  top  of  the  blast-furnace  stack, 
and  less  fuel  needs  to  be  burned  in  the  furnace  to  main- 
tain a  given  temperature. 

Increased  capacity,  because,  since  less  coke  is  charged, 
ore  and  flux  may  take  its  place. 

Grayer  pig  iron.  —  The  higher  temperature  in  the  hot 
blast-furnace  favors  the  reduction  of  silica,  and  the  pres- 
ence of  silicon  in  the  iron  causes  a  large  part  of  the  carbon 
to  crystallize  out  as  graphite,  i.e.,  it  renders  the  iron  gray. 

The  first  method  of  heating  the  blast  was  to  pass  it 
through  cast-iron  pipes,  which  were  enclosed  in  a  furnace 
and  maintained  at  the  highest  temperature  that  is  safe 
for  the  material  of  the  pipes.  The  temperature,  however, 
is  only  about  900°  F.  In  order  that  a  higher  blast  tem- 
perature may  be  reached,  special  hot-blast  stoves  have 
been  designed.  The  Cowper  type  is  shown  in  Fig.  3. 
It  consists  of  a  cylindrical  shell  of  iron  plates  lined  with 
fire-bricks.  C  is  a  combustion-chamber,  and  D  is  a 
chamber  filled  with  "  chequer  work."  The  gas  fuel  from 
the  top  of  the  stack  passes  through  the  pipe  G,  the  dust- 
separator  H,  and  into  the  combustion-chamber  at  J. 
Here  it  meets  air  which  enters  at  A,  and  combustion 
takes  place.  The  heated  products  of  combustion  pass 
down  through  D  and  on  through  E  to  the  chimney.  This 
process  is  continued  until  the  combustion-chamber  and  the 
chequer  work  are  raised  to  the  temperature  of  combus- 
tion. In  the  meantime  the  air  from  the  blowing  engine 
enters  the  other  stove  (which  has  been  previously  heated) 
at  K,  passes  up  through  the  chequer-work  chamber,  and 
down  through  the  combustion-chamber.  The  air  gains 
heat  from  the  chequer  work  and  is  thereby  raised  to  a 
temperature  somewhere  between  1000°  and  1500°  F. 


48  MATERIALS  OF   MACHINES 

It  then  passes  through  L  and  P  to  the  tuyeres  T,  where  it 
enters  the  furnace.  When  this  stove  is  cooled  so  that  the 
blast  is  insufficiently  heated,  properly  arranged  valves  are 
changed,  and  the  gas  burns  in  the  stove  at  the  left,  while 
the  blast  enters  through  the  stove  at  the  right. 

Since  the  action  of  the  blast-furnace  is  continuous, 
there  must  be  at  least  three  stoves,  so  that  any  one  may 
be  put  out  of  service  for  cleaning  or  repairs. 

There  are  still  some  blast-furnaces  that  use  charcoal 
as  fuel,  with  cold  blast.  The  product  is  white  iron. 
Because  of  the  lower  temperature  in  the  furnace  less  silica 
is  reduced,  and  less  silicon  is  absorbed  by  the  iron.  Be- 
cause of  the  small  amount  of  silicon  the  carbon  combines 
with  the  iron,  instead  of  separating  as  graphite,  and  the 
iron  fracture  is  white.  This  iron  is  used  for  chilled  car- 
wheels,  malleable  cast  iron,  etc. 

Dry  blast.*  —  It  has  been  known  for  many  years  that 
the  variation  of  moisture  in  the  air  seriously  affects  the 
operation  of  the  blast-furnace.  The  air-blast  enters  the 
furnace  and  strikes  the  white  hot  material  of  the  descend- 
ing charge  including  the  coke.  The  heat  in  this  part  of 
the  furnace  is  increased  by  combination  of  oxygen  of  the 
air  with  carbon  of  the  coke;  but  it  is  decreased  by 
the  sensible  heat  required  to  raise  the  temperature  of  the 
blast  to  the  furnace  temperature.  If  the  air  is  free  from 
water  vapor,  only  nitrogen  and  oxygen  (in  its  combination 
with  carbon)  absorb  heat;  but  if  the  air  carries  water 
vapor  this  also  has  to  be  heated.  Although  the  percent- 
age by  weight  of  the  vapor  in  the  air  is  small,  yet  its  heat 
capacity  —  specific  heat  —  is  more  than  twice  as  great 
as  that  of  nitrogen,  and  hence,  high  humidity  of  the  air 
may  chill  the  furnace  and  interfere  with  satisfactory 

*  See  Journal  Iron  and  Steel  Institute,  Vol.  LXVI,  1904,  Part  II, 
page  274. 


METALLURGY  OF  IRON   AND  STEEL  49 

operation.  Of  course,  with  any  amount  of  vapor  the 
temperature  in  the  furnace  could  be  maintained  if  there 
were  coke  enough  in  the  portion  of  the  charge  where  the 
blast  enters;  but  the  water  vapor  in  the  air  is  subject  to 
sudden  and  irregular  variations,  and  it  is  impossible  to 
adjust  the  quantity  of  coke  at  the  tuyeres  to  meet  these 
variations  because  of  the  long  time  required  for  the  de- 
scent of  the  charge.  The  ideal  condition  is  with  air  for 
the  blast  with  a  uniform  minimum  of  water  vapor. 

Mr.  James  Gayley  designed  a  plant  for  furnishing 
" dry-air  blast,"  which  was  constructed  and  applied  to 
the  Isabella  furnaces  of  the  Carnegie  Steel  Company  at 
Etna,  Pa.,  and  started  August  11,  1904.  Mr.  Gayley's 
scheme  is  as  follows:  The  air  on  its  way  to  the  blowing 
engine  passes  through  a  chamber  containing  coils  of  pipe 
that  are  kept  at  a  low  temperature  through  the  agency 
of  an  ammonia-compression  refrigerating  plant.  The 
moisture  in  the  blast  air  is  deposited  on  the  coils  as  water 
or  frost;  and,  after  the  accumulated  frost  has  reached  a 
certain  thickness,  the  refrigerating  medium  flowing  in  the 
coils  is  shut  off  and  a  hot  medium,  which  melts  the  frost, 
is  forced  through  in  its  stead.  The  resulting  water  is 
drawn  off,  the  refrigerating  conditions  are  restored  and 
frost  begins  to  form  again.  The  frost-melting  process  is 
applied  to  a  few  coils  at  a  time  and  does  not  interfere 
with  continuous  running. 

The  introduction  of  this  dry-air  blast  makes  it  possible 
to  increase  the  burden  —  the  weight  of  ore  per  ton  of 
coke  —  by  an  amount  equal  to  20  per  cent  or  more  with 
proportionate  decrease  in  cost  of  fuel  per  ton  of  product. 
Also,  since  the  air  after  passing  the  drying  chamber 
reaches  the  blowing  engines  at  a  reduced  temperature 
and  increased  density,  the  weight  of  air  delivered  per 
engine  stroke  is  increased  and  the  engine  speed  is  reduced 


50  MATERIALS  OF  MACHINES 

with  corresponding  gain  in  power  cost.  In  the  experi- 
mental plant  this  gain  in  power  in  the  blowing  engines  was 
greater  than  the  power  required  to  drive  the  refrigerating 
plant.  But  the  greatest  gain  from  Mr.  Gayley's  invention 
results  from  the  power  of  control  that  it  gives  over  con- 
ditions of  operation.  It  becomes  possible  as  never  here- 
tofore to  turn  out  different  grades  of  pig  iron  at  will,  and 
thus  to  command  the  most  profitable  market. 

Pig  iron  from  the  blast-furnace  goes  either  (a)  to  the 
foundry  to  be  converted  into  castings,  or  (6)  to  the  pud- 
dling mill  to  be  converted  into  wrought  iron,  or  (c)  to 
the  Bessemer  mill  to  be  converted  into  Bessemer  steel 
or  (d)  to  the  open-hearth  furnace  to  be  converted  into 
open-hearth  steel. 

In  the  foundry  pig  iron  is  melted,  with  very  little 
chemical  change,  and  poured  into  sand  molds,  where  it 
solidifies  in  the  required  form.  This  material  is  called 
cast  iron. 

The  pig  iron  is  melted  in  a  cupola-furnace.  See  Fig. 
5.  This  consists  of  a  plate-iron  shell  lined  with  fire- 
brick and  supported  upon  standards.  Double  doors,  A, 
opening  downward,  are  closed  and  held  in  position  by  a 
prop,  P,  and  a  sand  bottom  is  rammed  into  place  with  a 
slope  toward  the  tapping-hole  T.  The  top  of  the  cupola 
is  open. 

A  fan  or  blower  supplies  air-blast  at  a  pressure  of  from 
5  to  10  ounces  per  square  inch.  The  air  enters  through 
the  pipe  B  and  passes  into  the  furnace  by  way  of  the  cham- 
ber C  and  the  openings  E. 

The  charge  is  elevated  to  a  platform,  indicated  at  F, 
and  is  introduced  into  the  furnace  through  the  charging- 
door  D.  Kindling  and  wood  are  first  laid  upon  the  sand 
bottom.  Upon  this  the  "bed  "  of  coke  is  charged,  and 
then  alternate  layers  of  iron  and  coke  until  the  level  of  the 


METALLURGY   OF  IRON  AND  STEEL 


51 


charging-door  is  reached.  The  fire  is  lighted  and  the 
blast  turned  on.  The  coke  burns,  and  the  iron  melts; 
and  as  the  top  of  the  charge  settles  gradually,  more  iron 
and  coke  are  "  charged  on." 

The  melted  iron  collects  in  the  bottom  and  is  drawn  off 
periodically  at  T  into  a  receiv- 
ing ladle,  from  which  it  is  dis- 
tributed. Since  the  hot  iron 
comes  in  contact  with  the 
air-blast  there  is  always  silica 
produced  by  the  oxidation  of 
some  of  the  silicon.  Also,  a 
considerable  amount  of  silica 
sand  is  introduced  into  the 
cupola  adhering  to  the  pig 
iron.  If  the  cupola  only  runs 
one  or  two  hours  a  day,  as  in 
small  foundries,  the  silica  does 
not  interfere  with  operation. 
But  for  long  or  continuous  run- 
ning it  is  necessary  to  include 
limestone  with  the  charge  for 
a  flux,  and  to  tap  off  slag  at  S. 

After  all  the  iron  to  be  melt- 
ed has  been  charged  into  the 
cupola,  the  drawing  off  of  melt- 
ed iron  continues  and  the 
charge  settles  down  until  the 
cupola  is  empty  except  for  slag 
and  a  little  iron. 


FIG.  5. 
The  blast  is  then  stopped,  the  prop  P 


is  knocked  out,  the  doors  A  swing  down,  and  the  residue 
of  slag  and  iron  drops  out. 

Puddling  process.  —  Both  pig  iron  and  wrought  iron 
contain  silicon,   manganese,  carbon,  sulphur  and  phos- 


52 


MATERIALS  OF  MACHINES 


phorus;  but  in  pig  iron  the  sum  of  these  is  usually  from 
3  to  10  per  cent,  while  in  wrought  iron  their  sum  does  not 
usually  exceed  1  per  cent.  The  object  of  puddling  is  to 
change  pig  iron  into  wrought  iron.  The  process  must, 
therefore,  provide  means  for  the  removal  of  a  part  of  these 
substances.  The  removal  is  effected  by  oxidation  and  the 
puddling  process  is  carried  on  in  a  reverberatory  furnace. 
This  furnace  requires  description. 

See  Fig.  6.  A  is  a  fire-box  provided  with  a  grate  upon 
which  solid  fuel  is  burned.  H  is  a  hearth  in  which  the 
metallurgical  operation  is  carried  on.  E  is  a  passage 


FIG.  6. 

connecting  with  the  stack.  F  is  the  ash-pit,  and  B  and  D 
are  doors  for  the  introduction  of  fuel  and  the  material 
to  be  treated  in  the  hearth.  The  material  in  the  hearth 
is  heated  by  the  hot  gases  which  pass  over  it,  and  also  by 
heat  reflected  from  the  highly-heated  refractory  material 
of  the  furnace  roof.  Solid  fuel  burns  on  the  grate,  and 
the  air-supply  through  the  ash-pit  is  under  control.  If 
air-supply  were  just  sufficient  for  perfect  combustion, 
the  resulting  carbon  dioxide  and  nitrogen,  at  the  temper- 
ature of  combustion,  would  pass  over  the  hearth  and  give 
up  part  of  their  heat  to  the  furnace  walls,  and  to  the  ma- 


METALLURGY  OF  IRON  AND  STEEL  53 

terial  in  the  hearth,  and  then  go  on  at  lower  temperature 
to  the  stack.  But  if  air-supply  is  more  restricted,  carbon 
monoxide  will  result,  which  will  burn  in  the  hearth  with 
air  admitted  above  the  fire  or  through  the  bridge-wall  L. 
In  this  case  the  fire-box  becomes  a  gas-producer,  and  the 
gas  burns  in  the  hearth. 

The  flame  which  passes  over  the  hearth  of  this  furnace 
may  be  made  an  oxidizing,  a  neutral  or  a  reducing 
flame.  Thus,  by  a  free  admission  of  air  through  the  fire 
or  above  it,  complete  combustion  of  all  carbon  monoxide 
is  insured,  and  an  excess  of  oxygen  is  carried  over  the 
hearth  with  the  products  of  combustion.  This  results  in 
a  tendency  to  oxidize  materials  in  the  hearth.  If  the  ad- 
mission of  air  is  so  regulated  as  to  supply  only  just  enough 
oxygen  to  complete  the  combustion  of  all  carbon  monoxide, 
the  flame  will  be  neutral,  i.e.,  it  will  not  tend  either  to 
give  out  or  to  take  up  oxygen.  If  the  air-supply  is  re- 
stricted below  the  fire,  carbon  monoxide  will  result  from 
the  incomplete  combustion;  and  if  no  air  is  admitted 
above  the  fire,  this  carbon  monoxide  will  tend  to  take  up 
oxygen  from  the  materials  in  the  hearth  or  to  reduce 
them. 

In  the  form  of  reverberatory  furnace  used  for  puddling, 
the  bottom  of  the  hearth  is  made  up  of  cast-iron  plates 
which  are  covered  to  a  depth  of  about  three  inches  with 
a  lining  or  " fettling"  composed  of  silica  and  oxide  of 
iron.  The  fettling  is  put  in  as  follows :  tap-cinder  (which 
may  be  represented  thus:  2  FeO,  Si02)  is  charged  in  upon 
the  iron  plates,  spread  evenly,  and  subjected  to  a  tem- 
perature high  enough  to  soften  it  in  the  presence  of  oxygen. 
The  FeO  takes  up  oxygen  and  becomes  Fe2O3.  This  will 
not  remain  in  combination  with  the  silica,  and  hence,  the 
fusible  silicate  is  converted  into  infusible  ferric  oxide  and 
silica.  Then  scrap  iron  is  charged  in  and  subjected  to 


54  MATERIALS  OF   MACHINES 

an  oxidizing  flame.  It  is  thereby  changed  to  magnetic 
oxide,  which  is  raised  to  a  welding  heat  and  spread  smoothly 
over  the  hearth  bottom. 

If  the  hearth  were  lined  with  silica,  the  lining  would  be 
fluxed  away  by  the  ferrous  oxide  formed  during  the  pud- 
dling process,  with  considerable  loss  of  iron.  Also,  it  is 
impossible  to  remove  phosphorus  in  the  presence  of  free 
silica.* 

There  are  two  puddling  processes:  dry  puddling  and 
wet  puddling.  In  the  first,  and  less  used  process,  white 
pig  iron  is  heated  in  the  hearth  of  the  reverberatory  fur- 
nace and  subjected  to  the  action  of  an  oxidizing  flame. 
White  iron  differs  from  gray  iron  in  passing  through  an 
intermediate  pasty  condition  before  melting.  During 
the  passage  through  this  condition,  the  iron  is  constantly 
stirred  with  a  "  rabble  "  or  iron  bar,  which  is  inserted 
through  a  hole  in  the  door  D.  The  order  in  which  oxida- 
tion of  substances  occurs  is  silicon,  manganese,  carbon, 
iron.  A  considerable  part  of  the  silicon  and  manganese 
is  oxidized  during  the  melting,  and  ferrous  oxide  also  is 
formed.  The  silica  and  manganese  oxide  combine  to 
form  silicate  of  manganese,  a  fusible  slag,  and  if  silica  is 
still  left  free  it  combines  with  ferrous  oxide  to  form  silicate 
of  iron,  also  a  fusible  slag.  When  the  silicon  and  manga- 
nese are  completely  oxidized,  the  oxygen  attacks  the  car- 
bon and  iron  at  the  surface  of  the  bath  of  metal.  The 
resulting  carbon  dioxide  passes  off  to  the  stack,  and  ferrous 
oxide  acts  as  a  carrier  of  oxygen,  i.e.,  it  is  mixed  with  the 
bath  and  gives  up  its  oxygen  to  combine  with  the  carbon 
of  the  iron  carbide,  and  the  result  is  that  carbon  mon- 
oxide bubbles  up  to  the  surface  of  the  bath  and  burns 
there  to  carbon  dioxide,  while  the  iron  of  the  oxide  and 
carbide  remains  in  the  hearth.  This  continues  until  the 
*  See  page  61. 


METALLURGY  OF  IRON  AND  STEEL  55 

carbon  is  almost  entirely  removed.  Then,  because  of  the 
raising  of  the  fusing-point,  the  iron  begins  to  solidify  and 
is  collected  in  a  " puddle  ball,"  which  is  really  a  sponge 
of  iron  with  its  interstices  filled  with  slag.  This  is  raised 
to  a  welding  temperature,  and  put  through  a  squeezer, 
where  the  slag  is  squeezed  out  and  the  iron  is  welded  into 
a  " bloom."  This  bloom  is  then  put  through  a  " roughing 
train"  of  rolls  and  is  thereby  converted  into  "muck 
bar,"  which  is  cut  up,  piled,  reheated,  welded  under  a 
hammer  and  rolled  into  "merchant  bar."  This  piling, 
heating  and  rolling  is  sometimes  repeated,  with  a  result- 
ing product  of  finer  fiber  and  increased  strength  and 
ductility. 

In  wet  puddling,  the  more  commonly  used  process, 
gray  iron  is  used,  and  it  is  allowed  to  become  entirely 
fluid  before  it  is  "rabbled."  The  oxide  of  iron  in  this 
process,  instead  of  being  formed  in  the  furnace,  is  de- 
rived from  the  fettling  or  is  introduced  in  the  form  of 
"mill  scale,"  *  or  from  slag  of  previous  heats  that  is  rich 
in  ferrous  oxide  or  from  some  kind  of  rich  ore. 

The  chief  distinction,  then,  between  the  two  puddling 
processes  is  that  in  dry  puddling  the  oxygen  is  supplied 
by  the  air,  while  in  wet  puddling  the  oxygen  comes  from 
the  oxide  of  iron  which  is  introduced  with  the  pig  iron. 

In  order  that  the  phosphorus  may  be  removed  it  is 
necessary  that  there  should  be  an  excess  of  ferrous  oxide 
in  the  fettling  and  the  slag.  Then  phosphorus  is  oxi- 
dized to  P2O5,  and  this  combines  with  FeO  to  form  ferrous 
phosphate,  Fe3P2O8.  This  is  the  form  in  which  the  phos- 
phorus appears  in  the  slag.  If  there  had  been  uncom- 
bined  silica  present  in  the  slag  the  phosphoric  anhydride 
would  have  been  reduced  again  to  iron  phosphide  and 

*  The  iron  oxide  that  scales  off  from  iron  when  it  is  hammered 
or  rolled. 


56  MATERIALS  OF   MACHINES 

the  phosphorus  would  have  appeared  in  the  iron  instead 
of  in  the  slag. 

Sulphur  is  removed  in  the  puddling  process,  but  the 
manner  of  its  removal  is  not  clearly  understood.  The 
sulphur  exists  in  the  pig  iron  as  iron  sulphide,  and  it 
appears  in  the  slag  in  the  same  form.  A  basic  slag  (i.e., 
slag  with  excess  of  ferrous  oxide),  and  a  long  period  of 
contact  of  iron  with  the  slag,  are  favorable  to  the  removal 
of  sulphur. 

A  process  is  sometimes  used  which  is  intermediate 
between  the  blast-furnace  process  and  the  puddling 
process.  It  is  called  refining.  It  removes  most  of  the 
silicon  and  manganese,  but  stops  the  process  of  removal 
before  the  iron  becomes  too  infusible  to  be  cast.  The 
furnace  for  this  process  is  a  rectangular  hearth  with 
tuyeres  on  two  sides  bringing  air  under  pressure.  Melted 
iron  from  the  blast-furnace  may  be  run  into  this  furnace 
and  subjected  to  the  oxidizing  air-blast,  or  pig  iron,  may 
be  charged  in  with  coke  to  melt  it.  In  either  case  iron 
oxide  may  be  introduced  to  hasten  the  removal  of  silicon 
and  manganese.  The  iron,  after  completion  of  the  treat- 
ment, is  drawn  out  into  sand  molds,  where  it  cools  in 
the  form  of  plates.  These  plates  are  broken  up  and 
converted  into  wrought  iron  in  the  puddling-furnace.  The 
refinery  changes  gray  pig  iron  into  white  pig  iron,  because 
it  removes  the  silicon,  which  causes  much  of  the  carbon 
to  change  into  graphite  during  cooling. 

Processes  for  making  tool-steel  from  wrought  iron.  — 
The  difference  between  wrought  iron  and  tool-steel  is 
chiefly  in  the  amount  of  carbon  contained. 

Wrought  iron  has  from  0.1  per  cent  to  0.3  per  cent. 

Tool-steel  has  from  0.5  per  cent  to  1.5  per  cent. 
The  change  from  wrought  iron  to  tool-steel  is,  therefore, 
to  be  effected  by  addition  of  carbon. 


METALLURGY  OF  IRON  AND  STEEL  57 

Cementation  Process.  —  Bars  of  very  pure  wrought 
iron,  about  f  inch  by  5  inches  by  12  feet  long,  are  packed 
in  refractory  boxes  about  3  feet  wide  by  3  feet  deep, 
with  alternate  layers  of  rather  finely  divided  charcoal. 
These  boxes,  which  are  sealed  up  to  exclude  the  air,  are 
placed  in  a  furnace,  where  the  temperature  is  gradually 
raised  to  a  maximum  of  2000°  F. ;  this  temperature  is 
maintained  for  several  days,  and  then  the  furnace  is  al- 
lowed to  cool  down.  Iron  in  contact  with  carbon  at  a 
high  temperature  tends  to  absorb  carbon  slowly,  and  it  is 
found  that  the  bars,  after  treatment  as  described,  are 
changed  to  steel.  The  carbon,  however,  is  not  uniformly 
distributed,  the  structure  is  coarse,  because  of  long-con- 
tinued high  temperature,  and  the  material  is  brittle. 
This  material  (called  blister-steel)  is  changed  to  tool- 
steel  by  the  crucible  process,  as  follows: 

Crucible  process.  —  The  blister-steel  is  broken  up 
into  small  pieces  and  charged  into  refractory  crucibles 
about  2  feet  high,  with  an  average  diameter  of  about 
10  inches.  These  crucibles  are  placed  in  a  furnace, 
usually  of  the  Siemens'  regenerative  type,  where  the 
melting  temperature  of  steel  can  be  attained,  and  their 
contents  can  be  fused.  This  fluid  steel  is  then  cast  into 
an  ingot,  which  is  homogeneous  chemically,  but  of  coarse, 
crystalline  structure,  because  of  its  heat  treatment. 
It  is  then  reheated  and  hammered  into  standard  sizes 
and  forms,  and  the  mechanical  working  gives  it  a  fine 
homogeneous  structure. 

The  cementation  process  is  often  omitted  and  wrought 
iron  is  charged  into  the  melting  crucibles  together  with 
cast  iron  free  from  sulphur  and  phosphorus  to  furnish 
carbon.  Coke  or  charcoal  may  be  charged  also  to  pre- 
vent oxidation  at  the  surface,  and  to  serve  as  a  source 
of  carbon.  Some  carbon  may  be  absorbed  from  the  cru- 


58  MATERIALS  OF   MACHINES 

cibles  which  contain  either  graphite  or  finely  divided 
coke.  Either  ferromanganese  or  spiegeleisen  *  is  intro- 
duced into  the  crucible,  because  the  manganese  reduces 
any  iron  oxide  that  may  be  present,  and  removes  gas  or 
causes  it  to  go  into  solution,  thus  preventing  porosity. 
The  carbon  of  the  ferro  or  spiegel  increases  the  carbon 
of  the  steel.  The  melter  regulates  these  sources  of  car- 
bon so  as  to  insure  close  approximation  to  the  required 
grade  of  the  product. 

The  Bessemer  process.  —  Bessemer  steel  is  very 
similar  to  wrought  iron  in  chemical  composition,  but 
usually  contains  a  little  more  carbon.  The  structure, 
however,  is  different,  because  of  the  difference  in  the 
method  of  manufacture.  Thus  wrought  iron  is  built  up 
from  small  particles  of  iron  covered  with  slag.  The  slag 
is  not  entirely  removed  and  the  process  of  rolling  draws 
out  the  particles  into  threads  that  are  still  surrounded  by 
slag.  This  gives  wrought  iron  the  appearance  of  a  fibrous 
structure.  But  Bessemer  steel  is  cast  into  a  solid  ingot 
and  then  drawn  down  to  the  required  shape  and  size. 
It,  therefore,  shows  the  crystalline  structure  of  the  iron 
itself. 

The  Bessemer  process  changes  pig  iron  into  steel  con- 
taining from  0.1  per  cent  to  0.6  per  cent  of  carbon.  This 
change  is  accomplished  in  a  vessel  called  a  converter. 
See  Fig.  7. 

The  Vessel  is  made  up  of  riveted  iron  or  steel  plates, 
and  is  lined  with  "ganister."f  The  converter  is  mounted 
upon  trunnions  A,  A,  and  can  be  turned  about  the  axis  of 
the  trunnions  into  any  required  position.  Cold  air  from 
a  blowing  engine,  at  a  pressure  of  from  20  to  25  pounds 
per  square  inch,  enters  at  E,  follows  the  passage  shown 

*  See  page  43. 
f  See  page  33. 


METALLURGY  OF  IRON  AND  STEEL 


59 


to  Fj  whence  it  passes  into  the  converter  through  holes 
about  f  inch  diameter  that  pierce  the  conical  fire-bricks 
shown  in  the  converter  bottom. 

The  Bessemer  plant  includes  cupolas  for  melting  the 
pig  iron.  The  melted  iron  is  conveyed  to  the  converters 
either  through  properly-formed  channels  with  refractory 
linings,  or  in  ladle-cars  running  upon  a  track.  Sometimes 


FIG.  7. 


these  cars  transport  the  fluid  iron  directly  from  the  blast- 
furnace to  the  converter. 

The  converter  is  turned  on  its  side,  and  a  charge  of 
iron  is  run  in.  It  is  then  turned  into  a  vertical  position, 
a  valve  opens  automatically  to  turn  on  the  blast,  and 
the  air  is  forced  through  the  bath  of  iron.  The  results 
are  as  follows: 

The  oxygen  of  the  air  combines  with  the  oxidizable 
substances  of  the  bath;  and,  iron  being  in  excess,  ferrous 
oxide  is  formed  throughout  the  entire  "blow."  But 
ferrous  oxide  is  reduced  by  silicon  that  is  present  with 


60  MATERIALS  OF   MACHINES 

formation  of  silica.  Silica  is  also  formed  by  direct 
combination  of  silicon  with  oxygen  of  the  air.  Manga- 
nese also  is  present  and  oxide  of  manganese  is  formed; 
this  combines  with  silica  to  form  silicate  of  manganese, 
a  fusible  slag.  If  the  silica  is  in  excess,  some  fusible 
silicate  of  iron  is  also  formed.  During  this  period  bril- 
liant sparks  of  slag  are  thrown  from  the  mouth  of  the 
converter. 

When  all  the  silicon  and  manganese  are  removed,  the 
carbon  begins  to  be  oxidized,  directly  by  the  oxygen  of 
the  air,  and  indirectly  by  the  oxygen  of  the  ferrous  oxide. 
Carbon  monoxide  is  formed,  which  passes  off  from  the 
bath,  and  on  reaching  the  mouth  of  the  converter  burns 
to  carbon  dioxide  in  a  long  flame.  When  the  oxidation 
of  the  carbon  is  complete,  there  is  no  substance  left  to 
reduce  the  iron  oxide  formed,  reddish  fumes  appear  at 
the  mouth  of  the  converter,  and  the  process  is  immedi- 
ately stopped  by  turning  the  converter  on  its  side. 

The  converter  now  contains  nearly  pure  iron,  and, 
although  its  fusion  temperature  is  about  2900°  F.,  it 
remains  fluid.  The  fuel  which,  by  its  oxidation  or  com- 
bustion, has  raised  the  temperature  of  the  converter  from 
the  melting-point  of  pig  iron  to  that  of  wrought  iron,  is 
the  silicon,  manganese  and  carbon  of  the  pig  iron. 

When  the  first  experiments  were  made  on  the  Bessemer 
process,  it  was  thought  that  the  blow  could  be  stopped 
at  the  right  point  to  leave  the  amount  of  carbon  neces- 
sary to  make  steel;  but  it  was  impossible  to  get  a  uni- 
form product,  and  the  resulting  metal  was  brittle  and 
worthless. 

This  was  because  iron  oxide  remained  in  the  metal, 
and  because  gas  was  occluded,  causing  porosity.  To 
overcome  these  difficulties,  the  blow  is  continued  until  the 
carbon  is  completely  removed,  and  a  known  proportion 


METALLURGY  OF  IRON  AND  STEEL  61 

of  spiegeleisen  or  ferromanganese  *  is  added  to  effect  the 
recarburization.  The  manganese  reduces  the  iron  oxide, 
and,  in  some  not  very  well  understood  way,  removes  the 
occluded  gases  or  causes  them  to  go  into  solution.  After 
the  addition  of  the  spiegel  or  ferro,  the  steel  is  poured 
from  the  converter  into  a  ladle,  from  which  it  is  cast  into 
ingots,  which  are  rolled  into  rails,  or  plates,  or  into  blooms 
which  are  to  be  rolled  or  forged  into  required  forms. 

The  basic  Bessemer  process.  —  During  the  blow  as 
described,  phosphoric  acid  and  ferrous  oxide  are  formed 
simultaneously,  and  these  combine  to  form  phosphate 
of  iron,  or  ferrous  phosphate;  thus  3  FeO  +  P2O5  = 
FesP208.  But  this  is  reduced  again  to  iron  phosphide 
by  silicon  and  carbon,  and,  therefore,  little  or  no  phos- 
phorus can  be  removed  until  after  the  complete  removal 
of  these  substances  from  the  metal  in  the  converter. 
Ferrous  phosphate  is  also  reduced  by  silica,  because  the 
silica  has  greater  affinity  for  ferrous  oxide  than  phosphoric 
acid  has,  and  so  ferrous  silicate  is  formed  and  phos- 
phoric acid  is  left,  which  is  probably  reduced  to  iron 
phosphide  by  the  metallic  iron,  with  formation  of  ferrous 
oxide. 

The  lining  of  the  Bessemer  converter  described  is 
largely  silica,  and,  therefore,  silica  is  always  present,  and 
no  phosphorus  can  be  removed  in  a  converter  with  a 
ganister  or  acid  lining.  It  is  necessary,  therefore,  to  use 
for  this  process  pig  iron  which  is  very  low  in  phosphorus, 
since  the  presence  of  phosphorus  in  the  product  in  any 
considerable  amount  is  very  undesirable. 

The  fact  that  a  large  proportion  of  the  iron  ore  of  the 

world  contains  phosphorus,  which  is  not  removed  in  the 

blast-furnace,  made  it  desirable  to  find  a  way  to  eliminate 

phosphorus  in  the  steel-making  process.     This  led  to  the 

*  See  page  43. 


62  MATERIALS  OF   MACHINES 

invention  of  the  basic  Bessemer  process,  in  which  a  lining 
of  lime  and  magnesia  is  substituted  for  ganister  in  the 
converter.  The  only  free  silica,  then,  is  that  which 
results  from  the  oxidation  of  the  silicon  in  the  pig  iron. 
This  combines  with  lime,  which  is  charged  into  the  con- 
verter before  the  blow,  and  forms  a  stable  slag,  the  silica 
being  thereby  rendered  powerless  to  reduce  the  ferrous 
phosphate. 

The  lime  or  magnesia  present  then  replaces  the  ferrous 
oxide  of  the  ferrous  phosphate,  forming  calcium  or  mag- 
nesium phosphate,  which  is  probably  the  form  in  which 
the  phosphorus  chiefly  exists  hi  the  slag. 

In  the  acid  process  iron  is  not  usually  used  which  con- 
tains less  than  2  per  cent  of  silicon,  because  the  combustion 
of  at  least  that  amount  of  silicon  is  necessary  to  produce 
a  sufficiently  high  temperature  in  the  converter. 

In  the  basic  process  silicon  is  an  undesirable  element, 
since  all  the  silica  produced  must  be  neutralized  by  lime, 
in  order  that  the  process  shall  succeed.  For  this  reason 
iron  with  0.5  per  cent  silicon  is  best,  and  1.5  per  cent  is 
the  highest  allowable  limit.  This  makes  it  necessary  to 
substitute  some  other  fuel,  and,  therefore,  pig  iron  high 
in  manganese  is  used.  The  phosphorus,  usually  present 
from  a  minimum  of  1.5  per  cent  to  3  per  cent,  is  also  a 
fuel  and  raises  the  temperature  during  the  "afterblow." 
In  the  basic  process  little  or  no  phosphorus  is  removed 
until  after  the  complete  removal  of  the  carbon,  and  the 
blow  has  to  be  continued  after  the  " dropping"  of  the 
carbon  flame.  The  duration  of  the  afterblow  is  deter- 
mined from  a  knowledge  of  the  amount  of  phosphorus  in 
the  pig  iron  used,  or  by  taking  samples  at  intervals  dur- 
ing the  afterblow  and  making  physical  tests. 

The  steel  after  the  afterblow  of  the  basic  bessemer 
process  must  not  be  recarburized  in  the  converter,  be- 


METALLURGY  OF  IRON  AND  STEEL  63 

cause,  in  the  presence  of  the  spiegeleisen  or  ferromanga- 
nese,  the  phosphorus  compounds  of  the  basic  slag  tend 
to  be  reduced,  the  phosphorus  liberated  returning  to  the 
iron.  Hence,  the  iron  and  slag  in  the  converter  are  sep- 
arated as  completely  as  possible  by  (1)  pouring  off  the 
liquid  slag  from  the  iron  and  then  (2)  pouring  the  iron 
into  a  ladle,  leaving  the  partly  solidified  basic  slag  in 
the  converter.  Then  the  fluid  iron  in  the  ladle  freed  from 
slag  is  recarburized  in  the  usual  way  by  the  addition  of 
spiegeleisen  or  ferromanganese.  The  iron  is  oxidized 
to  a  greater  extent  in  the  basic  than  in  the  acid  blow,* 
and  hence,  more  manganese  is  required  to  deoxidize  the 
iron. 

The  best  pig  iron  for  the  basic  process  contains: 

Per  cent 

Phosphorus about       3 

Manganese over         2 

Silicon about       0.5 

Sulphur less  than  0.1 

This  is  white  iron,  because  of  high  manganese  and  low 
silicon,  whereas  the  high  silicon  iron  used  in  the  acid 
process  is  gray. 

Control  of  temperature  in  the  Bessemer  converter.  - 
Either  too  high  or  too  low  temperature  of  the  steel  at 
pouring  results  in  porosity,  and,  therefore,  this  temperature 

*  Possibly  because  in  the  acid  process  the  time  for  stopping  the 
blow  is  indicated  definitely  by  the  appearance  of  the  red  fumes  of 
iron  oxide,  whereas  the  time  for  stopping  the  blow  in  the  basic  process 
can  only  be  determined  by  the  knowledge  of  the  amount  of  phos- 
phorus in  the  charge  and  knowledge  of  the  rate  of  removal,  checked 
by  tests  of  the  bath  for  phosphorus.  Hence,  the  tendency  to  over- 
blow in  the  basic  is  greater  than  in  the  acid  process,  with  the  proba- 
bility of  a  greater  amount  of  free  oxygen  and  iron  oxide  in  the  blown 
metal. 


64  MATERIALS  OF  MACHINES 

must  be  carefully  regulated.  If  iron  too  high  in  silicon 
is  used  in  the  acid  process,  too  high  temperature  results, 
and  conversely. 

In  the  basic  process  the  difficulty  is  usually  to  keep 
the  temperature  high  enough.  If  the  temperature  is  too 
high,  it  may  be  reduced  by  charging  in  scrap-steel  from 
the  mill,  which  is  thus  remelted,  absorbing  surplus  heat, 
and  is  rendered  available  for  use.  The  temperature  is 
also  sometimes  reduced  by  admitting  a  small  amount  of 
steam  into  the  blast-pipe. 


FIG.  8. 

If  the  temperature  becomes  too  low,  the  converter  may 
be  inclined,  as  shown  in  Fig.  8,  during  the  burning  out 
of  the  carbon.  When  the  converter  is  vertical  the  carbon 
monoxide  formed  burns  at  the  mouth  of  the  converter, 
and  the  heat  evolved  is  lost  as  far  as  raising  the  tempera- 
ture inside  of  the  converter  is  concerned.  In  the  in- 
clined position,  however,  a  part  of  the  air  of  the  blast 
passes  through  the  metal  bath  and  forms  carbon  mon- 
oxide, and  a  part  passes  through  the  uncovered  tuyere 
holes  and  furnishes  oxygen  to  the  carbon  monoxide,  and 
carbon  dioxide  is  formed;  i.e.,  combustion  occurs  inside 


METALLURGY  OF  IRON  AND  STEEL 


65 


of  the  converter,  and  the  heat  developed  raises  the  tem- 
perature of  the  metal  bath. 

Graphical  representation  of  the  basic  Bessemer  proc- 
ess.—  Fig.  9  is  copied  from  Wedding's  "Basic  Bessemer 
Process,"  *  page  143.  The  diagram  is  plotted  from  the 
results  of  experiments  and  shows  the  history  of  a  blow 
in  a  basic  converter.  Horizontal  distances  from  0  rep- 
resent time,  each  division  corresponding  to  one  minute. 
Vertical  distances  from  0  represent  percentages  of  the 
substances  to  be  removed.  Therefore,  the  curves  represent 
the  change  in  percentage  of  the  substances  during  the 
blow. 

The  silicon  is  reduced  very  rapidly  from  1.2  per  cent  at 
the  beginning  of  the  blow,  and  after  six  minutes  only 

TIME,  MINUTES..  r 

2      3      I      5      6      7      8      9      10    11     12     13     U     15    16     17    18  '  19 


3.3 
3.2 
3.1 
3.0 
2.9 
2.8 
2.7 
2.6 
2.5 
2.4 
2.3 
2.2 
2.1 
2.0 
1.9 
1.8 
1.7 
1.6 
1.5 
1.4 
1.3 
1.2 
1.1 

BM* 

0.8 
0. 

O.G 
0. 
0.4 
0.3 
0.2 
0.1 
0 


FIG.  9. 


*  Translated  by  Phillips  and  Prochaska;  E.  &  F.  N.  Spon,  pub- 
lishers, London. 


66  MATERIALS  OF   MACHINES 

0.1  per  cent  remains.  From  this  point  on  the  silicon  is 
slowly  reduced  to  zero. 

The  manganese  is  reduced  less  rapidly  than  the  silicon, 
changing  from  1.05  per  cent  at  the  beginning  to  0.15 
per  cent  after  nine  minutes.  It  remains  nearly  constant 
during  the  carbon  reduction,  and  then  becomes  less  than 
0.1  per  cent. 

There  is  but  little  change  in  the  carbon  until  most  of 
the  silicon  is  removed,  when  the  curve  drops  rapidly, 
and  the  removal  is  practically  complete  in  sixteen  min- 
utes. Up  to  this  time  there  has  been  little  change  in  the 
phosphorus.  This  is,  of  course,  because  the  ferrous  phos- 
phate is  reduced  by  carbon.  From  this  point  the  removal 
of  phosphorus  is  very  rapid,  being  practically  complete 
after  the  blow  has  continued  twenty  minutes. 

The  curve  of  sulphur  was  shown  on  the  original  diagram, 
but  it  was  not  copied,  as  the  quantity  of  sulphur  remained 
nearly  constant,  its  value  being  less  than  0.1  per  cent. 
The  blow  ends  at  the  twenty-minute  line,  and  the  curves 
beyond  show  the  effect  of  introducing  spiegeleisen. 

Fig.  10  *  gives  the  history  of  an  acid  Bessemer  blow. 
The  amount  of  silicon  is  very  low  for  the  acid  process. 
Phosphorus  remains  practically  constant  at  0.1  per  cent, 
and  sulphur  at  0.06  per  cent.  Figs.  9  and  10  are  plotted 
on  the  same  scale  for  comparison.  The  blow  ends  at 
9  minutes  10  seconds,  and  the  rest  of  the  curve  results 
from  the  introduction  of  spiegeleisen. 

Open-hearth  processes.  —  Steel  is  also  made  from 
pig  iron  in  the  hearth  of  a  Siemens'  regenerative  furnace, 
(see  Fig.  2).  The  silicon,  manganese  and  carbon  are 
removed  by  oxidation,  as  in  the  puddling,  or  in  the  Bes- 

*  Plotted  from  experiments  of  F.  Julian  at  the  South  Chicago 
works  of  Illinois  Steel  Company.  See  H.  M.  Howe,  Journal  Iron 
and  Steel  Institute,  Vol.  11,  1890,  page  102. 


METALLURGY  OF  IRON  AND  STEEL 


67 


semer,  process.  Two  processes  were  formerly  carried 
on  in  open-hearth  furnaces:  first,  Siemens,  or  "pig  and 
ore,"  process;  second,  Siemens-Martin,  or  "pig  and 
scrap,"  process.  •  These  are  combined  into  a  single  process 
in  modern  practice,  and  both  scrap  and  ore  are  used. 

In  the  Siemens  process  pig  iron  is  charged  into  the 
hearth  and  melted,  part  of  the  silicon  and  manganese 

TIME,  MINUTES. 
2      3      4      6      «      7 


Q 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

V 

\ 

\ 

\ 

\ 

\ 

s^ 

\ 

Sis 

\ 

\ 

\ 

\ 

\. 

k 

\ 

\ 

\ 

\ 

\ 

/ 

Mn" 

x 

\ 

/ 

X 

\ 

V 

/ 

[U 

^  —  -j 

.  ^»» 



= 

^^=. 

•>_  - 

FIG.  10. 

being  oxidized  during  the  melting,  and  then  rich  ore  is 
added  to  supply  the  oxygen  to  combine  with  the  remain- 
ing silicon  and  manganese,  and  the  carbon  of  the  iron 
carbide.  When  the  action  is  complete  the  bath  of  nearly 
pure  iron  is  recarburized  by  the  addition  of  spiegeleisen 
or  ferromanganese,  and  the  manganese  reduces  the  fer- 
rous oxide  present,  and  removes  occluded  gases  or  causes 
them  to  be  dissolved,  as  in  the  Bessemer  process. 

In  the  Siemens-Martin  process  pig  iron  is  charged  into 
the  hearth,  and  melted  with  partial  removal  of  the  silicon 


68  MATERIALS  OF  MACHINES 

and  manganese,  and  then  steel  scrap  is  charged  into  the 
bath,  which  melts,  and  thus  the  percentage  of  silicon, 
manganese  and  carbon  is  reduced  by  dilution.  The 
remaining  part  of  these  substances  is  removed  by  the 
direct  action  of  the  oxidizing  flame,  and  the  indirect 
action  of  the  ferrous  oxide  formed  at  the  surface  of  the 
bath,  and  mixed  with  it.  Spiegel  or  ferro  are  added 
as  in  the  Siemens  process.  Ferrosilicon  and  ferroalumi- 
num  are  sometimes  used  in  place  of  ferromanganese 
for  the  recarburization,  the  removal  of  iron  oxide  and  the 
prevention  of  porosity. 

Either  acid  or  basic  lining  may  be  used  in  the  furnace 
in  which  the  open-hearth  processes  are  carried  on.  With 
the  acid  lining  no  phosphorus  is  removed  and  hence  low 
phosphorus  pig  must  be  used.  With  the  basic  lining 
the  phosphorus  is  removed  as  in  the  basic  Bessemer 
process. 

Duplex  process.  —  In  modern  practice  in  large  steel 
plants  the  Bessemer  converter  and  the  open-hearth  furnace 
are  sometimes  operated  in  combination.  A  charge  is 
"blown"  in  an  acid  converter  to  remove  silicon  and 
manganese  and  a  portion  of  the  carbon;  the  charge  is 
then  transferred  to  a  basic  open  hearth  where  the  re- 
mainder of  the  carbon  and  the  phosphorus  are  removed. 
This  reduces  the  time  in  the  open  hearth  and  increases 
the  output,  and  the  converter  may  be  lined  with  ganister, 
which  is  much  cheaper  and  more  durable  than  a  basic 
lining.  Another  advantage  is  that  the  silica,  formed  by 
burning  the  silicon  of  the  charge,  is  excluded  from  the 
basic  furnace  where  phosphorus  is  removed,  and  hence 
does  not  need  to  be  fluxed  with  lime;  the  result  is  economy 
of  lime  and  increased  durability  of  the  open-hearth  lining; 
this  method  also  makes  it  possible  to  use  a  pig  iron 
high  both  in  silicon  and  phosphorus,  which  would  be  un- 


METALLURGY  OF   IRON  AND  STEEL  69 

desirable  either  in  the  basic  converter  or  basic  open  hearth 
used  separately,  because  of  expense  for  lime  and  difficulty 
of  slag  disposal. 

Ductile  castings.  —  Many  machine  members  of  some- 
what complicated  form  need  to  be  strong  and  ductile. 
If  such  parts  were  required  in  large  numbers,  they  could 
usually  be  produced  by  the  process  of  casting,  much 
cheaper  than  by  the  process  of  forging.  For  this  reason 
much  attention  has  been  given  to  the  production  of 
ductile  castings.  The  most  important  resulting  processes 
are  those  for  the  production  of  malleable  castings  and 
steel  castings.  Some  of  the  grades  of  brass  and  bronze 
give  castings  which  are  strong  and  ductile,  but  the  high 
cost  puts  them  out  of  competition  for  many  purposes. 

Malleable  castings.  —  Castings  of  the  required  form 
are  made  of  iron  that  cools  with  all  of  its  carbon  in  the 
combined  state.  These  castings,  which  have  a  white 
fracture,  and  are  hard,  weak  and  brittle,  are  packed  in 
cast-iron  boxes  surrounded  with  coarsely  divided  oxide 
of  iron,  usually  hematite  ore  or  hammer  scale.  These 
boxes  are  sealed  and  brought  to  a  temperature  of  full 
redness,  from  1500°  to  1600°  F.,  as  quickly  as  possible  in 
a  reverberatory  oven  and  are  held  at  this  temperature  at 
least  60  hours,  and  are  then  cooled  very  slowly,  the  slow 
cooling  being  quite  important.  Cast  forms  are  thus  pro- 
duced that  are  very  much  like  wrought  iron  in  strength, 
ductility,  resilience  and  softness.  This  process  is  called 
mallifying  or  annealing.  Two  changes  occur  to  produce 
this  result:  first,  the  total  carbon  is  reduced;  second, 
the  combined  carbon  is  nearly  all  changed  to  graphitic 
carbon  or  "  temper  graphite,"  as  it  is  called. 

First  change.  —  The  reduction  of  total  carbon  in  the 
mallifying  process  occurs  chiefly  near  the  surface  of  the 
casting.  It  is  probable  that,  at  the  temperature  of 


70  MATERIALS  OF  MACHINES 

mallifying,  carbon  of  the  surface  iron  unites  with  oxygen 
of  the  iron  oxide,  or  with  oxygen  of  the  imprisoned  air, 
forming  gaseous  carbon  monoxide  or  carbon  dioxide  which 
passes  off;  there  is  then  a  tendency  to  movement  of 
carbon  from  the  inner  portions  of  the  casting  toward  the 
surface  where  it  in  turn  may  Combine  with  oxygen  and 
be  removed.*  The  quantity  of  carbon  thus  removed 
must  be  a  function  of  temperature,  time  of  exposure  to 
the  decarbonizing  conditions  and  of  distance  from  the 
surface  of  the  casting  to  the  middle.  Therefore,  with  a 
given  time  for  annealing,  thin  castings  might  be  quite 
transformed  as  to  physical  qualities,  while  thick  castings 
might  be  only  slightly  changed.  The  removal  of  carbon 
by  this  means  results  in  the  formation  of  a  surface  layer 
which  shows  a  wrought-iron-like  fracture,  while  the  rest 
of  the  fracture  is  black  because  of  the  second  change. 
It  is  probable  that  migration  of  carbon  occurs  to  only  a 
slight  extent  in  the  ordinary  American  mallifying  process. 
Second  change.  — When  molten  cast  iron  with  high  car- 
bon content  cools  under  ordinary  conditions,  part  of  the 
combined  carbon  becomes  graphitic  carbon,  appearing 
as  flakes  distributed  throughout  the  cooled  gray  iron. 
The  proportion  of  carbon  thus  becoming  graphitic  in- 
creases with  the  increase  of  total  carbon,  with  the  increase 
in  time  of  cooling,  with  increase  in  silicon  and  decreases 
with  increase  in  manganese;  see  page  127.  But  when 
iron  contains  less  than  3  per  cent  of  carbon  with  ordi- 
nary rate  of  cooling  and  with  low  silicon,  the  molten  iron 
cools  white,  all  carbon  being  in  combination.  If  this 
white  iron  is  raised  to  a  temperature  of  from  1500°  to 
1600°  F.  as  in  the  mallifying  process,  the  combined  carbon 

*  The  fact  of  movement  of  carbon  in  solid  iron  at  high  tempera- 
tures is  shown  by  the  case-hardening  process  and  by  the  cementation 
process;  see  pages  172  and  57. 


METALLURGY  OF  IRON  AND  STEEL  71 

tends  to  change  into  temper-graphite,  and  if  the  tempera- 
ture is  maintained  long  enough  the  change  may  be  quite 
complete.  The  graphite  thus  produced  takes  the  form 
of  very  minute  particles  nearly  uniformly  distributed, 
which  interrupt  the  continuity  of  the  iron  structure 
much  less  than  the  graphite  flakes  in  gray  iron;  its  pres- 
ence, therefore,  has  less  effect  to  reduce  strength  and 
ductility  than  the  graphite  of  the  gray  iron  castings. 
During  the  change  of  combined  carbon  into  temper 
graphite  the  resultant  effect  is  to  increase  strength  and 
ductility,  because  reduction  of  combined  carbon  has 
much  greater  influence  in  increasing  these  qualities  than 
the  presence  of  the  temper  graphite  has  to  reduce  them. 
The  first  change  is  probably  more  effective  in  increasing 
ductility,  but  it  can  only  be  satisfactorily  accomplished 
in  light  castings.  The  second  change  occurs  in  heavier 
castings  and  increases  ductility,  though  in  less  degree 
than  the  first  change.  This  may  be  made  clearer  by  an 
experiment;*  a  casting  having  one  portion  about  f  inch 
thick  and  another  portion  about  T3g-  inch  thick  was  put 
through  the  mallifying  process.  Tests  of  the  mallified 
castings  for  carbon  showed  results  as  follows : 


Form  of  carbon 

Thick  part 

Thin  part 

Graphite  ... 

Per  cent 
2  93 

Per  cent 

1.72 

Combined  carbon  

0.04 

0.20 

Total  carbon. 

2  97 

1.92 

There  was  a  much  greater  reduction  of  total  carbon  in  the 
thin  part,  but  a  more  complete  conversion  of  combined 
carbon  into  temper  graphite  in  the  thick  part.  The  thin 
part  was  much  more  ductile  than  the  thick  part. 

*  This  experiment  was  made  at  the  works  of  the  Westmoreland 
Malleable  Iron  Co. 


72 


MATERIALS  OF   MACHINES 


During  the  early  years  of  development  and  use  of  this 
process  the  iron  for  the  castings  was  white  pig  iron  from 
cold-blast  charcoal  furnaces,  because  low  carbon  and  low 
silicon  were  necessary  for  success  in  the  mallifying  process. 
This  iron  was  melted  in  regular  foundry  cupolas  with 
very  little  chemical  change.  But  in  present  practice  gray 
pig  iron  from  coke  furnaces  is  melted  in  a  reverbera- 
tory  furnace,  called  an  "air  furnace,"  and  the  fluid  iron 
is  subjected  to  an  oxidizing  flame  until  silicon,  manganese 
and  carbon  are  reduced  as  low  as  is  consistent  with  fluidity 
necessary  for  the  production  of  "  sharp  "  castings. 

The  changes  that  occur  in  very  light  castings  during 
the  mallifying  process  are  shown  by  the  following  average 
results  of  several  analyses: 


Condition  of  castings 

Total  carbon 

Graphitic 
carbon 

Combined 
carbon 

Before  mallifying  
After  mallifying  

Per  cent 

2.79 
1.74 

Per  cent 

0.177 
1.565 

Per  cent 
2.613 
0.175* 

*  Analyses  made  by  Mr.  W.  H.  McCord  at  the  chemical  laboratory  of  Stanford 
University. 

Reduction  of  total  carbon  in  the  castings  for  the  malli- 
fying process  is  sometimes  accomplished  by  charging 
steel  or  wrought-iron  "  scrap  "  into  the  cupola  or  air 
furnace;  this  reduces  the  carbon  by  dilution. 

Long  experience  has  shown  that  the  iron  charged  into 
the  air  furnace  should  contain  about  the  following  per- 
centages of  the  substances  given: 

Per  cent 

Silicon 1.25 

Manganese 0.40 

Phosphorus 0.15 toO.2 

Sulphur,  not  over 0.05 

Carbon.  .  .  3.5. 


METALLURGY  OF  IRON  AND  STEEL  73 

In  the  furnace  the  silicon  is  reduced  to  from  0.7  per  cent 
to  1  per  cent;  manganese  and  sulphur  are  only  slightly 
changed,  while  carbon  is  reduced  to  about  2.75  per  cent. 
The  phosphorus  is  not  removed,  but  its  presence  in  nearly 
the  specified  amount  is  desirable,  because  it  renders  the 
molten  iron  more  fluid  for  casting  and  does  not  harm  the 
product. 

If  it  is  necessary  to  heat  malleable  castings  for  any 
purpose,  as  for  straightening  or  bending,  great  care  is 
necessary  because,  if  the  temperature  is  raised  much 
above  the  mallifying  temperature,  from  1500°  to  1600°  F. 
the  temper  graphite  combines  again  with  the  iron  and  the 
castings  become  brittle  as  they  were  before  mallifying. 

Steel  castings.  —  Some  cast  machine  members  need 
to  be  stronger  and  more  ductile  than  malleable  castings; 
other  cast  machine  members  are  too  thick  to  mallify 
satisfactorily.  To  meet  such  cases  steel  castings  are  made 
by  "pouring  molten  steel  into  molds  directly  from  the 
steel-making  process.  When  this  is  done  the  steel  foun- 
dry includes  steel  making  as  well  as  steel  founding. 

Converters  and  open-hearth  furnaces  are  used  in  steel 
foundries  though  they  are  usually  somewhat  smaller  than 
those  producing  steel  ingots  in  steel  mills.  Special  con- 
verters are  used  like  the  Tropenas  or  the  Stoughton  types.* 
In  these  converters  all  or  part  of  the  air  enters  above  the 
surface  of  the  metal  bath  and  oxidation  of  the  substances 
to  be  removed  occurs  by  direct  combination  with  oxygen 
of  the  air  and  by  indirect  combination  with  the  oxygen 
of  the  iron  oxide,  which  is  formed  and  mixed  with  the 
bath  and  reduced  again  throughout  the  blow.  Obviously, 
the  carbon  is  burned  to  carbon  dioxide  (see  page  64) 
and  the  entire  heat  of  combustion  of  the  carbon,  is  util- 

*  See  "  The  Metallurgy  of  Iron  and  Steel  "  by  Bradley  Stoughton; 
McGraw-Hill  Book  Co.,  page  272. 


74  MATERIALS  OF  MACHINES 

ized  to  help  fix  the  temperature  of  the  bath  through  the 
necessary  range. 

Reheating  and  working  of  steel  ingots  by  mill  processes 
improves  strength  and  ductility ;  but  steel  castings  do  not 
get  the  benefit  of  this  improving  process,  and  hence,  the 
pig  iron  used  for  making  steel  for  castings  must  be  lower 
in  sulphur,  and  in  phosphorus  also  if  the  acid  process  is 
used,  than  that  used  for  ingots,  in  order  that  the  castings 
may  equal  the  forgings,  made  from  the  ingots,  in  strength 
and  ductility.  This  means  a  more  expensive  pig  iron, 
which  adds  to  the  cost  of  the  steel  castings. 

Basic  steel  is  more  apt  to  have  porous  spots  and  "blow 
holes  "  when  cast  than  acid  steel,*  and  these  defects  are 
especially  undesirable  in  steel  castings,  because  they  cannot 
be  welded  up  as  in  ingots.  They  may  be  a  hidden  source 
of  weakness  in  stress  members;  and  often  much  useless 
expense  for  labor  is  incurred  when  a  "last  cut  "  reveals 
porous  defects  that  lead  to  the  rejection  of  a  nearly 
finished  casting.  The  presence  of  manganese  or  silica 
tends  to  prevent  porosity,  and  one  or  other  of  these  sub- 
stances is  introduced  in  the  recarburization;  but  it  is 
difficult  to  hold  silicon  from  oxidization  during  "teem- 
ing "  or  casting,  which  occupies  more  time  for  steel  cast- 
ings than  for  ingots,  and  if  manganese  is  introduced  in 
sufficient  quantities  to  protect  the  iron  from  oxidation  it 
may  appear  in  the  product  in  sufficient  quantity  to  cause 
weakness  and  brittleness. 

To  avoid  porosity  and  brittleness,  due  to  the  presence 
of  oxygen  or  iron  oxide,  a  small  amount  of  pure  aluminum 
is  sometimes  introduced  into  the  melt  before  pouring; 
the  aluminum  combines  with  the  oxygen  and  is  almost 
entirely  removed  in  the  slag. 

Steel  for  castings  is  sometimes  obtained  by  melting 
*  See  page  63, 


METALLURGY  OF  IRON  AND  STEEL  75 

proper  mixtures  in  crucibles.  Obviously  the  fuel  and 
labor  cost  is  higher,  but  in  the  castings  there  is  less  lia- 
bility of  formation  of  blow  holes,  and  it  is  easier  to  control 
the  content  of  substances  other  than  iron  than  in  the  open- 
hearth  or  the  converter. 

Steel  that  is  cooled  from  a  molten  state  has  a  rather 
coarse  crystalline  structure;  this  can  be  transformed  into 
a  fine  structure  with  accompanying  gain  in  strength  and 
ductility  by  reheating  and  rolling  or  hammering  at  suit- 
able temperatures  (see  page  171)  or  by  suitable  heat 
treatment  without  mechanical  working  (see  page  168). 
In  case  of  steel  castings  the  refining  of  structure  must, 
of  course,  be  accomplished  without  mechanical  working. 

Steel  for  castings  must  be  poured  hotter  than  steel  for 
ingots,  since  in  the  former  case  the  metal  must  "run 
sharp,"  in  order  to  take  the  required  form  in  the  mold, 
whereas  this  is  of  little  importance  in  the  case  of  ingots. 
Because  of  this,  higher  grade  refractory  material  is  needed 
for  the  linings  in  the  steel  foundry,  and  cost  of  furnace 
repairs  is  higher,  than  in  the  steel  mill. 

The  molds  for  steel  castings  are  made  both  of  "green 
sand  "  and  of  "dry  sand";  but  the  sand  in  either  case 
must  be  more  refractory  than  that  used  in  molds  for 
iron  castings,  because  the  casting  temperature  of  steel 
is  probably  from  2800°  F.  to  3000°  F.,  while  that  for 
cast  iron  is  about  2400°  F. 

There  is  probably  a  difference  of  about  400°  F.  or  500°  F. 
between  the  temperatures  of  solidification  of  steel  and 
cast  iron,  and,  since  solid  shrinkage  begins  when  solidi- 
fication is  complete,  it  follows  that  shrinkage  is  greater 
in  steel,  and,  therefore,  greater  care  must  be  taken  both  in 
the  design  of  the  steel  castings  and  in  the  regulation  of 
the  rate  of  cooling  of  different  parts  of  the  casting  that 
have  different  thickness  of  cross  section.  The  shrink- 


76  MATERIALS  OF   MACHINES 

age  stresses  are  removed  if  the  casting  is  reheated  for  heat 
treatment  to  refine  structure.  See  page  168. 

Steel  castings,  like  steel  ingots,  are  made  with  varying 
carbon  content  to  meet  the  varying  demand  for  strength, 
ductility  and  capacity  for  shock  resistance.  Increase  in 
strength  obtained  by  increase  in  carbon  is  accompanied 
by  reduction  of  ductility;  the  converse  also  is  true. 
Shock  resistance  increases  both  with  strength  and  duc- 
tility, but  the  increase  is  greater  with  ductility  than  with 
strength,  and  therefore  there  is  usually  a  resultant  increase 
in  shock  resistance  with  reduction  of  carbon. 

Steel  castings  contain  from  0.15  per  cent  to  0.7  per  cent 
of  carbon  according  to  the  required  service.  About 
0.1  per  cent  to  0.35  per  cent  of  silicon  is  usually  present, 
the  lower  value  for  soft  castings  and  the  higher  value  for 
hard  castings.  This  amount  of  silicon  does  not  diminish 
toughness  of  the  metal  itself  and  its  presence  seems  to 
reduce  the  amount  of  iron  oxide  and  free  oxygen  present 
with  their  undesirable  effects.  Sulphur  and  phosphorus 
should  be  kept  below  0.05  per  cent  in  castings  that  are 
to  be  subjected  to  severe  stress. 


CHAPTER  V 

OUTLINE  OF  THE  METALLURGY  OF  COPPER,* 
LEAD,  TIN,  ZINC  AND  ALUMINUM 

Copper  is  found  in  nature  as  native  or  pure  copper, 
and  in  combination  with  many  other  substances.  The 
only  combinations  that  are  of  commercial  importance 
are  sulphides,  oxides,  carbonates  and  silicates.  Whether 
the  copper  is  pure  or  combined  it  is  usually  mixed  with 
earthy  "gangue,"  the  amount  of  copper  present  varying 
from  less  than  one  per  cent  to  15  per  cent  or  more. 

Copper  is  obtained  from  its  ores  either,  (1)  by  "wet 
methods,"  or  hydrometallurgy,  or  (2)  by  "dry  methods," 
or  smelting.  In  (1)  the  copper  is  taken  up  by  some  solvent 
and  leached  out  of  the  ore,  the  gangue  remaining  prac- 
tically unchanged;  in  (2)  there  is  partial  separation  of  the 
metallic  combinations  from  the  gangue  by  reason  of 
specific  gravity,  (a)  without  application  of  heat;  or  (6) 
with  the  ore  in  a  molten  state.  Only  dry  methods, 
which  produce  over  90  per  cent  of  the  copper  of  commerce, 
will  be  considered  here. 

The  copper  that  is  produced  by  these  methods  is  usually 
quite  impure  and  must  be  refined  either  by  the  smelting 
refining  or  the  electrolytic  refining  process,  or  by  both. 

Native  copper.  —  The  ore  of  the  Lake  Superior  dis- 
trict contains  from  0.5  per  cent  to  4  per  cent  of  copper 

*  For  fuller  treatment  of  this  subject  see  "Practice  of  Copper 
Smelting "  by  Peters  and  "  The  Metallurgy  of  the  Common 
Metals  "  by  Austin. 

77 


78  MATERIALS  OF  MACHINES 

rather  finely  divided  and  distributed  throughout  the 
earthy  material  of  wide  lodes.  This  ore  is  concentrated 
without  heat  until  the  copper  becomes  from  30  per  cent 
to  90  per  cent  or  more  of  the  "mineral  "  as  the  concen- 
trate is  called.  Ore  from  the  mine  is  crushed  and  passed 
through  steam  stamps  and  then  through  a  series  of  "  jigs," 
where  it  is  shaken  to  help  separation  by  gravity,  and 
where  a  stream  of  water  washes  away  the  lighter  portions 
from  the  upper  surface,  leaving  the  concentrated  mineral. 

This  mineral  is  smelted  in  a  reverberatory  furnace 
where  a  fusible  slag  is  formed  either  by  proper  mixture 
of  mineral  with  different  gangue  content,  or  by  introduc- 
tion of  a  suitable  flux.  Again  gravity  carries  the  heavier 
metal  down  and  the  fluid  slag  is  removed  and  the  copper 
is  cast  into  ingots. 

Oxides,  carbonates  and  silicates  of  copper.  These 
ores  are  found  in  the  upper  portions  of  mineral  deposits, 
having  been  formed  by  the  decomposition  of  copper  sul- 
phides through  the  agency  of  air  and  water.  The  smelt- 
ing of  such  ores  may  be  accomplished  in  a  furnace  of  the 
blast-furnace  type,  the  carbon  dioxide  of  carbonates  being 
driven  off  by  heat,  the  oxide  of  copper  being  reduced  by 
carbon  monoxide  from  the  fuel,  and  the  silica  of  silicates 
being  removed  in  the  slag.  The  gangue  enters  the  slag, 
which  is  made  fusible  by  proper  mixture  of  ores  or  by  the 
introduction  of  a  flux.  Molten  slag  and  molten  copper 
are  drawn  off  at  proper  intervals.  The  copper  loss  in  the 
slag  in  this  process  is  large,  because  copper  oxide  combines 
with  the  silica,  and  because  some  metallic  copper  is  carried 
away  mechanically.  This  process  is  not  very  extensively 
used  now  because  of  growing  scarcity  of  oxide  ores,  and 
also  because  it  is  advantageous  to  charge  oxide  ores  with 
sulphide  ores  to  help  supply  the  necessary  oxygen  for  the 
removal  of  sulphur  as  sulphur  dioxide. 


METALLURGY  OF  COPPER,   ETC.  79 

Copper  sulphide  ores.  —  The  sulphide  ores  of  copper 
contain  not  only  copper  sulphide  but  also  iron  sulphide 
together  with  the  gangue.  If  such  ore  is  melted  in  a 
neutral  or  reducing  atmosphere  the  copper  and  iron  sul- 
phide will  melt  into  a  "matte  "  which  settles  out  of  the 
ore  by  gravity,  leaving  the  gangue  which,  if  properly 
fluxed,  may  be  drawn  off.  No  sulphur  is  removed  by 
this  treatment;  but  if  the  ore  is  first  roasted  in  an  oxidiz- 
ing atmosphere  at  a  temperature  too  low  to  melt  the 
sulphides,  part  of  the  sulphur  will  burn  and  pass  off  as 
the  gas  S02.  Copper  holds  sulphur  more  strongly  than 
iron  does,  and  hence  the  iron  sulphide  yields  its  sulphur 
first  and  the  iron  deprived  of  its  sulphur  takes  up  oxygen. 
The  product  of  carefully  regulated  roasting  contains 
copper  sulphide,  iron  oxide  and  silicious  gangue ;  probably 
some  copper  oxide  has  been  formed  and  some  iron  sulphide 
remains.  When  this  product  is  smelted  the  copper  and 
iron  sulphides  melt,  carrying  down  any  precious  metals 
that  may  be  present;  the  iron  oxide  acts  as  a  flux  for  the 
silicious  gangue,  thus  forming  a  fusible,  removable  slag 
while  any  iron  sulphide  present  enters  the  matte  which 
is  drawn  off  and  cooled  as  "  coarse  metal."  In  this  proc- 
ess silica  sometimes  has  to  be  added  as  a  flux  when  the 
ore  carries  excess  of  iron  and  the  gangue  is  deficient  in 
silica.  The  roasting  and  smelting  are  repeated  until  all 
iron  and  gangue  are  removed  and  only  copper  sulphide, 
or  "fine  metal,"  remains.  This  fine  metal  is  cooled  in 
proper  form  and  size  and  is  roasted  in  an  oxidizing  atmos- 
phere; a  part  of  the  sulphur  burns,  while  the  copper  thus 
freed  from  sulphur  takes  up  oxygen,  forming  copper  oxide. 
When  this  roasting  has  continued  long  enough  to  produce 
the  right  proportions  of  copper  oxide  and  copper  sulphide, 
the  charge  is  melted  and  copper  oxide  gives  up  its  oxygen 
to  combine  with  the  sulphur  of  the  copper  sulphide,  pro- 


80  MATERIALS  OF  MACHINES 

ducing  sulphur  dioxide  which  passes  off  as  a  gas,  leaving 
metallic  copper. 

Blister  copper,  or  black  copper,  the  product  of  the 
three  processes  described,  still  contains  substances  that 
reduce  value  for  industrial  purposes;  sulphur,  iron  and 
copper  oxide  are  present;  and  there  often  are  precious 
metals,  gold,  silver  or  platinum,  that  may  be  removed 
with  profit;  and  often  antimony,  arsenic,  bismuth, 
selenium  and  tellurium.  These  five  substances,  if  present 
in  the  ore,  are  not  entirely  removed  by  the  roasting  or 
smelting  processes.  There  are  two  refining  processes: 
furnace  refining  and  electrolytic  refining.  If  it  is  found 
from  analysis  that  precious  metals  are  present  in  amounts 
that  will  make  their  recovery  profitable,  the  furnace 
refining  may  be  omitted  and  the  copper  cast  into  anode 
plates  which  are  transferred  to  the  electrolytic  refinery: 
here  they  are  suspended  in  an  electrolyte  consisting  of  a 
solution  of  copper  sulphate  in  dilute  sulphuric  acid.  A 
suitable  current  is  maintained  through  the  solution  and 
the  copper  is  transferred,  through  the  agency  of  the 
electric  energy,  from  the  anode  to  the  cathode,  where  it 
is  deposited  free  from  impurities.  The  precious  metals, 
together  with  the  other  substances  carried  by  the  anode 
plates,  fall  to  the  bottom  of  the  tank  containing  the  elec- 
trolyte, forming  "  slimes  "  which  are  treated  for  the  recov- 
ery of  the  precious  metals,  while  the  pure  copper  from  the 
cathodes  is  prepared  for  the  market. 

But  if  there  is  not  a  profitable  content  of  precious 
metals  the  refining  may  be  done  by  the  furnace  method. 
The  blister  copper  is  melted  down  in  a  reverberatory  fur- 
nace, a  slag  forming  during  the  melting,  which  is  probably 
composed  of  silica  present,  and  iron  oxide  formed  during 
the  melting;  this  slag  is  skimmed  off  and  compressed  air 
is  forced  through  the  bath  of  molten  metal;  the  result  is 


METALLURGY  OF  COPPER,   ETC.  81 

that  sulphur,  arsenic,  iron  and  copper  are  oxidized;  the 
oxidized  sulphur  and  arsenic  pass  off  as  gas,  and  the  iron 
oxide  combines  with  the  remaining  silica  to  form  addi- 
tional fusible  slag,  which  is  removed.  Some  of  the  cop- 
per oxide  formed  acts  as  a  carrier  of  oxygen,  yielding  its 
oxygen  to  combine  with  sulphur  of  any  copper  sulphide 
that  remains;  but  a  considerable  amount  of  copper  oxide 
is  left  in  the  bath  after  the  removal  of  impurities  is  com- 
plete. The  oxidizing  atmosphere  is  now  changed  to  a 
reducing  atmosphere  and  green  poles,  branches  of  trees, 
are  inserted  so  that  the  ends  dip  beneath  the  surface  of 
the  bath  of  molten  copper;  the  evolution  of  steam  and 
gas  agitates  the  bath  and  the  carbon  of  the  "charred  poles 
combines  with  the  oxygen  of  the  oxide  of  copper  and  forms 
carbon  monoxide,  leaving  the  pure  copper,  which  is  cast 
into  ingots. 

Lead.  —  Many  combinations  of  lead  with  other  sub- 
stances occur  in  nature,  but  almost  the  entire  supply  of 
lead  of  commerce  is  obtained  from  the  ore  galena  which 
consists  of  lead  sulphide  mixed  with  varying  proportions 
of  gangue.  Sometimes  the  galena  also  bears  silver  and, 
more  rarely,  gold,  while  copper,  antimony  and  arsenic 
are  commonly  present.  If  the  ore  does  not  contain  paying 
proportions  of  silver  or  gold,  it  may  be  concentrated 
without  heat  by  stamping  and  washing  before  melting. 
But  if  these  precious  metals  are  to  be  extracted  no  pre- 
liminary concentration  is  allowable  because  of  loss  of  the 
precious  metals  in  the  gangue  removed. 

Treatment  of  Galena.  Roasting.  —  Crushed  ore  is 
charged  into  a  reverberatory  furnace  and  a  low  tempera- 
ture is  maintained  in  an  oxidizing  atmosphere.  The 
sulphur  of  a  part  of  the  lead  sulphide,  PbS,  is  oxidized 
and  passes  off  as  sulphur  dioxide,  S02,  while  the  lead  that 
is  left  takes  up  oxygen,  forming  lead  oxide,  PbO;  also 


82  MATERIALS  OF   MACHINES 

some  lead  sulphate,  PbS04,  is  formed  by  oxidation  of  lead 
sulphide.     These  changes  may  be  represented  thus: 

PbS  +  3O  =  PbO  +  SO2. 
PbS  +  40  =  PbS04. 

Smelting.  —  After  the  roasting  has  continued  long 
enough  to  produce  proper  proportions,  the  lead  oxide  and 
sulphate  are  mixed  with  the  remaining  lead  sulphide, 
the  temperature  is  raised  in  a  neutral  atmosphere  and  the 
following  reactions  take  place: 


PbS  +  PbS04  =  2  Pb  +  2  S02. 

The  metallic  lead  melts  and  settles  in  a  low  part  of  the 
furnace,  whence  it  is  cast  into  ingots.  The  remainder  of 
the  charge  still  contains  a  large  amount  of  lead,  and  the 
roasting  and  smelting  are  repeated  several  times;  ulti- 
mately no  lead  sulphide  is  left  to  reduce  the  remaining 
lead  oxide  and  sulphate.  Then  fine  carbon  (coal)  is  intro- 
duced with  the  charge  to  take  away  oxygen  in  gaseous 
C02  from  the  lead  oxide,  and  to  reduce  lead  sulphate  to 
sulphide,  thus  rendering  the  sulphur  also  available  as  a 
reacting  agent.  Thus  an  additional  amount  of  lead  is 
freed,  melted  and  cast  into  ingots.  During  the  smelting 
lime  is  added  to  render  the  charge  more  resistant  to  melt- 
ing, since  it  is  desirable  to  melt  the  lead  out  from  the  solid 
residue.  In  this  residue  there  still  remains  a  considerable 
amount  of  lead,  probably  as  oxide  or  sulphate,  which 
cannot  be  removed  by  repetition  of  roasting  and  smelting; 
the  amount  may  run  as  high  as  30  per  cent.  Transfer 
may  then  be  made  to  a  blast-furnace  for  lead  smelting 
where  most  of  the  lead  may  be  recovered.  Each  of  the 
successive  smelting  processes  has  a  higher  temperature 
than  the  preceding  one  and  produces  less  pure  lead. 


METALLURGY  OF  COPPER,  ETC.  83 

This  process  is  not  suited  to  ores  containing  more  than 
5  per  cent  of  silica,  since  silica  unites  with  lead  oxide,  caus- 
ing loss  of  lead  in  the  slag.  Other  forms  of  furnace  are 
used  for  smelting  lead,  but  the  chemical  changes  are  the 
same  as  those  given,  the  difference  being  in  the  details  of 
operation. 

Ores  containing  lead,  silver  and  gold,  and  usually  copper, 
arsenic  and  antimony  are  smelted  in  a  blast-furnace,  an 
important  function  of  the  lead  being  to  collect  and  carry 
down  the  precious  metals  into  the  "base  bullion."  Oxide 
ores  of  lead  may  be  used  directly,  but  sulphide  ores  are 
roasted  so  that  the  resulting  oxide  may  be  reduced  by 
CO  of  the  blast-furnace;  some  sulphide  remains,  however, 
in  the  roasted  ore.  The  flux  used  is  iron  ore  and  lime- 
stone, and  the  ferric  oxide,  reduced  to  ferrous  oxide,  also 
acts  with  carbon  to  reduce  the  lead  sulphide,  as  follows: 

PbS  +  FeO  +  C  =  Pb  +  FeS  -f  CO 
and  also  to  reduce  the  lead  sulphate  as  follows: 

PbSO4  +  FeO  +  5  C  =  Pb  +  FeS  +  5  CO. 

The  molten  lead  not  only  carries  down  gold  and  silver,  but 
also  copper,  arsenic  and  antimony.  The  remaining  FeO 
unites  with  lime  of  the  flux  and  the  silica  of  the  gangue  to 
form  a  fusible  slag. 

Softening.  —  The  base  bullion  from  the  blast-furnace 
is  treated  first  in  a  shallow  reverberatory  furnace,  where  it 
is  melted  and  subjected,  in  an  oxidizing  atmosphere,  to 
heat  at  a  temperature  that  just  melts  the  lead.  At  the 
surface  lead,  copper,  antimony  and  arsenic  oxidize  and 
combine  into  a  "dross"  which  floats  upon  the  molten 
lead.  This  process  is  continued  until  tests  show  com- 
plete removal  of  the  copper,  arsenic  and  antimony. 
Then  the  charge  is  cooled  just  enough  to  harden  the  dross 


84  MATERIALS  OF  MACHINES 

which  is  skimmed  off;  the  lead  now  contains  only  silver 
and  gold,  and  is  treated  by  the  Pattinson  process. 

The  Pattinson  process  depends  upon  the  fact  that 
when  a  molten  mixture  of  lead  and  silver  is  cooled  slowly 
with  constant  stirring,  crystals  of  lead  very  low  in  silver 
will  form,  and  removal  of  these  crystals  leaves  lead  high 
in  silver.  This  process  is  carried  out  in  a  series  of  cast- 
iron  pots  set  in  masonry  with  properly  arranged  furnaces 
for  heating.  Lead  containing  silver  is  melted  in  the 
first  pot  and  then  the  source  of  heat  is  withdrawn;  as 
the  stirred  metal  cools  the  crystals  of  lead  very  low  in 
silver  are  dipped  into  the  second  pot  by  means  of  a  skim- 
mer that  allows  the  molten  metal  richer  in  silver  to  run 
back  into  the  first  pot.  Extension  of  the  process  eventu- 
ally gives  lead  very  high  in  silver  in  the  first  pot  and  lead 
very  low  in  silver  in  the  last  pot.  The  latter  goes  to  the 
lead  market,  while  the  former  is  treated  in  the  cupellation 
furnace,  where  the  lead  is  all  oxidized  into  PbO,  litharge, 
leaving  purified  silver.  Then  the  PbO  is  reduced  by  car- 
bon monoxide  in  a  reverberatory  furnace,  leaving  pure 
lead. 

The  Pattinson  process  leaves  copper,  arsenic  and  anti- 
mony with  the  silver  and  hence  produces  a  very  pure, 
soft  lead. 

Tin.  —  Nearly  all  the  tin  of  commerce  is  extracted 
from  the  ore  called  "  tinstone  "  or  "cassiterite,"  which 
consists  of  a  stannic  oxide,  Sn02,  mixed  with  gangue  con- 
taining earthy  and  metallic  substances. 

For  removal  of  the  earthy  portion  of  the  gangue  the 
ore  is  stamped  fine  and  washed  on  racks,  where  a  stream  of 
water  carries  away  the  lighter  material  from  the  surface, 
leaving  the  tin  oxide  and  other  heavy  material.  This 
concentrated  ore,  which  usually  contains  iron  and  copper 
pyrites,  FeS2  and  CuFeS2,  and  arsenical  pyrites,  FeSAs,  is 


METALLURGY  OF  COPPER,   ETC.  85 

roasted  in  an  oxidizing  atmosphere  in  a  reverberatory 
furnace.  Sulphur  is  removed  as  gaseous  SOz',  arsenic 
forms  the  oxide,  As203,  the  white  arsenic  of  commerce, 
which  is  caught  in  long  flues;  while  the  copper  becomes 
copper  oxide  and  copper  sulphate,  and  the  iron  becomes 
iron  oxide.  The  soluble  copper  sulphate  is  removed  from 
the  roasted  ore  by  washing.  The  washed  ore,  which  still 
contains  copper,  iron,  arsenic  and  sulphur,  is  then  reduced 
by  smelting  with  carbon  in  small  shaft  furnaces  or  in  rever- 
beratory furnaces.  In  either  case  oxygen  of  the  tin  oxide 
combines  to  form  carbon  dioxide  either  with  carbon  mixed 
directly  with  the  ore,  or  with  carbon  monoxide  from 
partial  combustion  of  coal  used  as  a  source  of  heat.  A 
small  amount  of  lime  is  used  as  a  flux  to  remove  silica 
that  may  remain.  The  ingots  of  crude  or  raw  tin  from 
the  smelting  process  still  carry  not  only  copper,  iron, 
arsenic  and  sulphur  but  often  also  lead,  antimony  and 
tungsten,  which  must  be  removed  by  refining.  This  refin- 
ing usually  consists  of  two  processes,  liquation  and  boiling. 
The  ingots  of  crude  tin  are  piled  on  the  hearth  of  a  rever- 
beratory furnace  and  the  temperature  is  slowly  increased 
to  a  point  where  the  tin  melts  out  leaving  the  unfused 
impurities.  The  molten  tin,  still  with  small  amounts  of 
iron,  arsenic  and  sulphur,  is  led  into  a  receptacle,  which 
has  a  separate  source  of  heat,  where  green  twigs  in  bundles, 
or  wet  sticks,  are  held  submerged  in  the  bath  of  tin.  Evo- 
lution of  steam  and  gas  causes  brisk  agitation  of  the  tin, 
whereby  all  parts  are  brought  in  contact  with  the  air  and 
the  impurities  are  oxidized.  A  scurn  of  a  portion  of  the 
oxides  thus  formed  collects  on  the  surface.  After  the 
boiling,  the  still  molten  metal  is  allowed  to  stand  for  about 
an  hour,  when  the  scum  is  removed  and  the  tin  is  ladled 
out  into  ingot  molds.  A  part  of  the  oxidized  impurities 
goes  out  with  the  scum,  but  another  part  which  is  of  higher 


86  MATERIALS  OF   MACHINES 

specific  gravity  than  the  tin  settles  toward  the  bottom. 
Thus,  when  the  tin  is  ladled  out,  that  which  comes  from 
the  top,  called  "  refined  tin,"  is  purer  than  the  so-called 
" common  tin"  from  the  lower  portions  of  the  mass. 
The  material  at  the  bottom  is  often  cooled  and  liquated 
and  boiled  again. 

The  residue  in  the  liquation  furnace  with  increased 
temperature  yields  more  tin  of  lower  grade. 

Sometimes  a  process  called  "  tossing  "  is  substituted 
for  boiling;  the  molten  tin  is  dipped  in  ladles  and  allowed 
to  run  back  into  the  bath  from  a  considerable  height. 

There  are  effective  methods  for  recovering  a  large  por- 
tion of  the  tin  which  remains  in  the  smelter  slags  and  in 
refining  dross. 

Zinc.  —  The  ores  from  which  zinc  of  commerce  is 
extracted  are: 

Zinc  blende,  which  consists  of  zinc  sulphide,  ZnS, 
mixed  with  earthy  gangue  and  usually  with  manganese, 
iron,  cadmium  and  silver;  more  rarely  it  bears  mercury, 
gold,  lead  and  tin. 

Calamine,  made  up  of  zinc  carbonate,  ZnCO3,  with 
cadmium,  iron  and  manganese  as  carbonates,  and  with 
lead  sulphide  and  iron  oxide,  and  earthy  gangue.  Zinc 
silicate,  Zn2Si04,  is  also  sometimes  present. 

Franklinite,  which  is  made  up  of  oxides  of  iron,  man- 
ganese and  zinc  with  earthy  gangue. 

Zinc  vaporizes  at  a  temperature  of  about  1725°  F.  and 
advantage  is  taken  of  this  fact  in  smelting.  Zinc  oxide 
and  coal,  both  finely  divided,  are  mixed  and  heated  in 
muffles  or  retorts,  where  the  zinc  oxide  is  reduced  by  the 
carbon  with  formation  of  carbon  monoxide,  and  where 
the  metallic  zinc  is  vaporized,  led  away  and  condensed. 

Since  the  ore  for  this  purpose  must  be  in  the  form  of 
zinc  oxide,  it  is  necessary  to  roast  the  ores  containing 


METALLURGY  OF  COPPER,   ETC. 


87 


zinc  sulphide  —  zinc  blende  —  in  an  oxidizing  atmos- 
phere to  oxidize  and  remove  the  sulphur  as  S02  and  to 
oxidize  the  zinc. 

Zinc  carbonate  ore  —  calamine  —  is  usually  roasted  to 
remove  moisture  and  to  drive  off  carbon  dioxide  from  the 
carbonate  in  order  to  produce  the  zinc  oxide  for  smelting. 

The  Belgian  process  for  smelting  zinc  is  chiefly  used 
in  the  United  States.  Retorts,  which  are  refractory 
cylinders  about  8J  inches  in  diameter  and  four  feet  long 
(see  Fig.  11)  are  set  in  tiers  in  a  chamber  having  a 
source  of  heat.  The  inner  end  of  the  retort  is  closed,  and 
a  refractory  cone  C  is  inserted  in  the  outer  open  end. 
The  retorts  are  charged 
with  the  fine  mixture  of 
coal  and  zinc  ore,  usu- 
ally moistened  so  as  to 
cohere,  and  the  heat  from 
the  furnace  raises  the 
temperature  of  the 
charge.  Water  is  driven 
off  as  steam  which  passes 
out  through  the  cone. 
The  carbon  of  the  coal 
unites  with  the  oxygen  of 
the  zinc  oxide  forming 
carbon  monoxide  which 
passes  through  the  outer  opening  of  the  cone  where  it 
burns — with  a  blue  flame  —  to  carbon  dioxide.  The  zinc 
thus  isolated  is  vaporized  and  the  vapor  passes  to  the  cone 
where,  on  meeting  the  cooler  walls,  it  is  condensed  and  the 
liquid  zinc  is  withdrawn  at  proper  intervals.  When  the 
charge  is  exhausted  the  cone  is  removed,  the  residue  is 
withdrawn,  the  retort  is  recharged,  the  cone  is  replaced 
and  the  process  begins  again. 


FIG.  11. 


88  MATERIALS  OF  MACHINES 

Aluminum.  —  Aluminum  occurs  very  abundantly  in 
nature,  but  it  is  always  in  combination  with  other  sub- 
stances, such  as  oxygen,  sodium,  fluorine  and  silicon. 

In  its  combinations  with  silicon  and  oxygen,  aluminum 
is  useful  for  refractories  and  in  the  ceramic  arts.  See 
Chapter  III. 

Metallic  aluminum  may  be  produced  by  several  methods, 
but  the  most  important  process  commercially  is  elec- 
trolysis, or  decomposition  by  an  electric  current,  of  alu- 
mina, Al20s.  Pure  alumina  is  very  infusible;  and,  since 
a  substance  must  be  fluid  for  electrolysis,  it  was  necessary 
to  find  a  solvent  for  alumina  that  would  melt  at  a  rela- 
tively low  temperature,  and  that  would  allow  the  decom- 
position of  the  alumina  without  being  affected  itself. 
In  1889  a  patent  was  granted  to  Charles  M.  Hall,  covering 
the  use  of  cryolite,  a  fluoride  of  aluminum  and  sodium, 
3  NaF,  A1F3,  as  a  solvent  bath  for  electrolysis  of  alumina. 
This  substance  melts  at  a  red  heat,  and  when  melted 
dissolves  alumina,  thus  forming  an  electrolyte  which  can 
be  decomposed  by  a  suitable  electric  current. 

The  process  is  carried  out  in  rectangular  cast-iron  pots 
having  a  lining  of  hard-baked  carbon  about  3  inches 
thick  which  forms  the  negative  electrode  or  cathode. 
The  positive  electrodes  or  anodes  consist  of  cylindrical 
carbons  about  3  inches  in  diameter  and  originally  about 
15  inches  long;  these  are  suspended  with  axes  vertical 
by  f-inch  rods  of  copper,  which  in  turn  are  clamped  to  a 
copper  bar  that  extends  above  the  pot  throughout  its 
entire  length.  This  bar  and  the  carbon  lining  of  the  pot 
are  connected  into  a  current  to  which  electrical  energy 
is  supplied  at  low  voltage.  The  anodes  are  lowered  until 
they  touch  the  cathode  which  is  the  lining  of  the  pot. 
This  completes  the  circuit  and  electricity  flows.  The 
anodes  are  then  withdrawn  slightly  to  form  an  air  gap, 


METALLURGY  OF  COPPER,  ETC.  89 

thus  providing  an  electric  furnace  of  the  arc  type.  Cryo- 
lite for  the  solvent  bath  is  then  introduced  and  is  melted 
by  the  heat  from  the  electrical  energy  supplied.  Pure 
alumina  is  then  stirred  into  the  bath  and  electrolysis 
begins.  The  A1203  is  decomposed  by  the  action  of  the 
current,  aluminum  being  deposited  on  the  cathode  or 
pot  lining,  while  oxygen  appears  at  the  surface  of  the 
anodes,  combines  with  the  anode  carbon  and  passes  off 
as  gaseous  carbon  dioxide,  thus  causing  the  anodes  to 
waste  away  and  to  require  periodical  renewal.  The  me- 
tallic aluminum  is  dipped  or  syphoned  out  at  intervals 
and  cast  into  ingots. 

The  alumina  for  this  process  must  be  free  from  other 
substances;  and,  since  pure  alumina  is  not  found  in  nature, 
a  purification  process  is  necessary.  The  source  of  the 
alumina  is  usually  bauxite,  which  has  already  been  de- 
scribed (see  page  34)  as  a  "  mixture  of  a  large  proportion 
of  hydrated  alumina,  A1203  •  2  H20,  with  clay,  silica,  iron 
oxide  and  titanic  oxide  and  often  with  another  hydrated 
aluminum  oxide,  Al203-3  H20."  The  separation  of  the 
alumina  from  the  other  substances  of  the  bauxite  is 
effected  as  follows: 

The  bauxite  is  ground  fine  and  mixed  with  fine  sodium 
carbonate;  the  mixture  is  stirred  and  heated  in  a  furnace 
and  the  alumina  displaces  the  carbon  dioxide  of  the  sodium 
carbonate,  driving  off  the  gaseous  carbon  dioxide  and 
forming  aluminate  of  soda.  This  may  be  expressed  chem- 
ically as  follows : 

Al203-3  H20+3  Na2CO3  =  Al2O3-3  Na2O+3  CO2+3H2O. 

The  aluminate  of  soda  thus  formed  is  soluble  in  water, 
while  the  other  substances  in  the  bauxite  remain  un- 
changed and  are  insoluble.  When  the  reaction  is  shown 
by  chemical  tests  to  be  complete  the  charge  is  withdrawn 


90  MATERIALS  OF   MACHINES 

and  cooled  and  the  aluminate  of  soda  is  taken  into  solu- 
tion in  warm  water  and  thus  separated  from  the  unde- 
sirable substances.  Then  carbon  dioxide  is  forced  through 
the  solution  of  aluminate  of  soda  and  soluble  sodium 
carbonate  is  formed,  and  the  alumina  which  is  precipi- 
tated is  filtered  out,  washed  and  dried  and  is  ready  for 
electrolysis.  The  sodium  carbonate  is  recovered  to  use 
again  in  the  process. 

Alumina  is  also  obtained  by  making  an  intimate  mix- 
ture [of  very  fine  cryolite  with  calcium  carbonate  (chalk) 
and  roasting  the  mixture;  the  chemical  reaction  is  as 
follows : 

2  (A1F8  -  3  NaF)  +  6  CaCO3  =  A12O3  •  3  Na^O 

+  6  CaF2  +  6  C02. 

The  products  are  gaseous  carbon  dioxide,  insoluble  calcium 
fluoride  and  aluminate  of  soda,  which  may  be  dissolved 
and  treated  as  in  the  last  process  to  produce  pure  alumina. 


PART   SECOND  — PHYSICAL  PROPER- 
TIES OF  MATERIALS 


CHAPTER  VI 
TESTING  MATERIALS 

MACHINE  members  in  service  are  subjected  to  the 
action  of  external  forces  which  tend  to  break  or  distort 
them.  Some  members,  like  springs,  fulfill  their  function 
by  yielding  periodically  through  considerable  space  to 
applied  forces;  but  a  large  proportion  of  members  require 
rigidity,  that  is,  the  yielding  under  applied  forces  must 
be  kept  very  small.  In  any  case  permanent  distortion 
and  breakage  must  be  prevented. 

The  designer  of  machines  must  know  the  effect  of 
external  forces  applied  to  the  materials  of  machines. 
This  knowledge  is  derived  from  tests.  A  test  piece  of 
suitable  dimensions  may  be  made  of  any  material  and  a 
steadily  increasing  force  may  be  applied  to  it  until  it 
breaks  or  is  very  much  deformed.  The  force  applied  to 
the  test  piece  may  tend  to  crush  it,  a  compressive  force, 
or  to  pull  it  apart,  a  tensile  force,  or  to  bend  it,  a  trans- 
verse force,  or  to  twist  it,  a  torsional  force,  or  the  external 
forces  may  produce  some  combination  of  these  tendencies. 

A  solid  resists  change  of  form;  forces  applied  to  a  solid 
tending  to  change  its  form  induce  stress  within  it; 
stress  may  be  defined  as  the  action  and  reaction  between 
adjacent  parts  of  a  solid  during  resistance  to  change  of 
form. 

91 


92 


MATERIALS  OF  MACHINES 


First  illustration.  In  Fig.  12,  suppose  the  tensile  force  P 
is  applied  to  a  cylindrical  test  piece;  in  any  cross  section 
like  AA  there  will  result  an  action  and  reaction  between 
adjacent  faces  of  the  section  resisting  separation;  this 
action  and  reaction  is  called  tensile  stress.  If  P  equals 


Fixed 
End 


A 

FIG.  12. 

5000  pounds  the  total  stress  in  the  section  AA  is  5000 
pounds;  if  the  cross-sectional  area  of  A  A  equals  one 
square  inch  the  unit  stress  in  the  section  is  5000  pounds 
per  square  inch.  If  the  area  of  cross  section  were  J  square 
inch,  the  unit  stress  would  equal  5000  -f-J  =  10,000  pounds 
per  square  inch.  Reversal  of  the  direction  of  the  external 
force  P  would  change  it  from  a  tensile  force  to  a  compres- 
sive  force  tending  to  crush  the  test  piece,  and  in  all  sec- 
tions like  AA  a  compressive  stress  would  result. 


FIG.  13. 

Second  illustration.  In  Fig.  13,  suppose  a  transverse 
force  P  applied  tending  to  bend  a  test  piece  of  rectangular 
cross  section.  If  the  test  piece  is  bent  by  this  force,  the 
fibers  below  a  neutral  axis  XX  would  be  stretched,  while 


TESTING   MATERIALS 


93 


those  above  would  be  shortened;  hence  the  bending  force 
would  produce  in  any  section  both  tensile  and  compressive 
stress.  Also  the  force  P  tends  to  cause  the  adjacent 
surfaces  in  any  section  like  AA  to  slide  over  each  other  and 
thus  there  is  produced  shearing  stress  in  the  section. 

Third  illustration.  The  test  piece  may  be  subjected  to  a 
force  that  tends  to  twist  it  about  its  axis,  as  in  Fig.  14. 
Then  in  any  section  AA  there  is  a  tendency  for  one  surface 
to  slide  over  the  adjacent  surface  about  the  axis  of  the 
test  piece,  and  hence  shearing  stress  is  induced. 

Testing  machines  have  been  designed  and  constructed 


Fixed 
End 


Applied  at  right  angles  to  paper 
P 


End  View 


FIG.  14. 


capable  of  applying  definite  forces  to  test  pieces  in  these 
several  ways,*  with  devices  for  measuring  the  defor- 
mation corresponding  to  any  applied  force.  In  whatever 
way  the  force  is  applied  to  the  test  piece  the  object  of 
the  test  is  to  record  simultaneous  values  of  stress  and 
deformation,  because  a  knowledge  of  the  relation  of  these 
values  enables  the  designer  to  proportion  machine  parts 
for  safety  from  breakage,  for  necessary  rigidity  in  opera- 
tion, and  for  economy  of  material. 

A  tension  test  of  ductile  material  will  now  be  consid- 
ered for  illustration.     The  increasing  tensile  force  P  is 


*  See  "  Experimental  Engineering,"  by  Carpenter  and  Diederichs, 
Chapter  IV. 


94  MATERIALS  OF  MACHINES 

applied  as  in  Fig.  12  and  the  resulting  change  of  form  — 
deformation  —  is  elongation,  accompanied  by  correspond- 
ing reduction  of  section  area.  Assume  that  P  is  applied 
in  successive  increments;  there  will  be  corresponding 
values  of  p,  representing  unit  stress  in  any  cross  section, 
and,  if  the  original  section  area  is  represented  by  F, 
the  successive  values  of  p  will  equal  the  corresponding 
values  of  P  ^  F* 

Assume  also  that  after  each  increment  of  stress  an 
accurate  measurement  of  elongation  is  made.  In  the 
early  part  of  the  test  the  elongation  is  proportional  to 
stress;  but  after  passing  a  certain  limit,  called  the  elas- 
tic limit,  the  elongation  becomes  increasingly  greater  for 
a  given  increase  in  stress.  The  law  may  be  stated  thus: 
deformation  is  proportional  to  stress  within  the  elastic 
limit. 

If,  before  reaching  the  elastic  limit,  the  stress  is  grad- 
ually reduced  to  zero,  the  elongation  becomes  zero  also; 
that  is,  the  test  piece  returns  to  its  original  dimensions. 
Within  this  limit  the  material  may  be  considered  per- 
fectly elastic,  f  since  elasticity  may  be  defined  as  the 
property  whereby  a  material  returns  to  its  original 
dimensions  on  relief  of  stress.  If,  however,  the  elastic 
limit  is  passed  before  relief  of  stress,  the  test  piece  will 
be  permanently  elongated.  This  permanent  elongation 

*  Since  F  decreases  with  increase  of  P  the  successive  values  of  p 
strictly  should  equal  the  corresponding  values  of  P  divided  by  the 
corresponding  values  of  varying  F.  In  practice,  however,  F  is  con- 
sidered constant  and  equal  to  the  original  section  area. 

f  It  is  probable  that  even  within  the  elastic  limit  the  material 
is  not  perfectly  elastic.  Very  refined  measurements  show  that 
materials  take  some  "  set"  even  under  relatively  small  stress.  The 
values  of  this  set,  however,  are  so  very  small  that  they  may  be 
safely  disregarded  in  the  ordinary  testing  of  the  materials  of  en- 
gineering. 


TESTING  MATERIALS  95 

is  called  "  set."  The  elongation  that  disappears  on  relief 
of  stress  is  called  elastic  deformation. 

After  passing  the  elastic  limit  the  elongation  accom- 
panying a  given  increment  of  stress  increases  steadily 
until  finally  the  maximum  stress  that  the  test  piece  is 
capable  of  sustaining  is  reached  and  rupture  occurs. 
This  maximum  stress  is  a  measure  of  the  ultimate  strength 
of  the  material. 

If  the  test  piece  were  divided  into  equal  units  of  length, 
say  one  inch,  as  in  Fig.  15,  and  if  the  material  were 
absolutely  homogeneous  and  of  equal  strength  in  all 
sections,  it  would  follow  that  with  increasing  stress  all 
units  of  length  would  share  equally  in  the  elongation 

|«- End  for  fastening  — >j< l-=  Portion  tested *{<-  End  for  fastening—^ 


FIG.  15. 

and,  on  reaching  a  stress  corresponding  to  the  ultimate 
strength  of  the  material,  all  sections  would  yield  at  once. 
But  no  such  material  is  available  for  machine  parts  and 
when  the  ultimate  strength  of  the  weakest  section  is 
reached,  local  yielding  occurs,  a  "neck  "  forms  and  the 
piece  breaks. 

Fig.  16  shows  a  tension  test  piece  before  and  after  test- 
ing. The  original  piece  is  subdivided  equally  by  punch 
marks  into  half -inch  units  of  length.  The  tested  piece 
shows  the  increase  in  length  of  these  units,  and  also  the 
local  reduction  of  area  and  fracture  at  minimum  section. 

As  the  test  goes  on  the  equal  divisions,  see  Fig.  15, 
share  the  elongation  almost  equally  until  maximum  stress 
is  reached.  At  any  point  in  the  test  there  is  a  total 
elongation  represented  by  X  expressed  in  inches  and  there 


Is? 


\S 


i 


:. 


; 
i 


+t 

i 


/         \ 


o 


t 


I 


TESTING  MATERIALS  97 

is  a  corresponding  unit  elongation  (or  elongation  of  each 
of  the  one-inch  divisions)  represented  by  c,  and  expressed 
in  inches  per  inch  of  original  length  I,  of  the  tested  section. 
It  follows  then  that 

_  X 

6  —  ~T  • 


After  passing  the  maximum  stress  the  elongation  and 
reduction  of  section  area  are  localized  at  or  near  the 
neck. 

The  simultaneous  values  of  unit  stress  p  and  of  relative 
elongation  e  may  be  plotted  with  reference  to  rectangular 
axes  (p  values  being  laid  off  vertically  upward  and  e 
values  being  laid  off  horizontally  toward  the  right)  and 
through  the  points  thus  located  a  curve  may  be  drawn 
called  the  stress-deformation  diagram.  In  what  fol- 
lows, this  name  will  be  abbreviated  to  s.-d.  diagram. 

Fig.  17  shows  s.-d.  diagrams  for  mild  steel.  Diagram  I 
is  plotted  on  such  a  scale  for  X  values  that  the  entire 
diagram  up  to  breaking  falls  within  the  limits  of  the 
figure.  The  yielding,  however,  on  this  scale  is  too  small  to 
show  until  a  unit  stress  of  nearly  33,000  pounds  is  reached. 
Hence  diagram  II  is  plotted  with  greatly  increased  scale 
for  values  of  c,  and  this  diagram  shows  the  test  only  as 
far  as  M,  diagram  I.  This  diagram  is  useful  as  showing 
more  clearly  what  occurs  in  the  early  part  of  the  test. 

Starting  from  A  with  stress  and  deformation  equal  to 
zero  the  line  of  the  diagram  is  straight  until  BI  is  reached. 
This  point  BI  is  the  elastic  limit  which  is  strictly  denned 
as  the  point  where  proportionality  of  stress  and  defor- 
mation ceases;  or,  otherwise  expressed,  it  is  the  point 
where  the  diagram  line  ceases  to  be  a  straight  line.  Usu- 
ally in  ductile  materials  the  diagram  line  then  curves  to 
the  right  until  the  yield  point  is  reached,  where  it  becomes 


98 


MATERIALS  OF  MACHINES 


TESTING  MATERIALS  99 

about  horizontal,  indicating  a  considerable  yielding 
without  increase  of  stress.*  At  C\  further  elongation 
requires  increase  of  stress  and  the  diagram  line  rises  in  a 
curve  to  M.  In  engineering  testing  the  yield  point  is 
usually  taken  at  the  elastic  limit. 

If  on  reaching  some  point  within  the  elastic  limit,  N,  for 
example,  stress  had  been  reduced  gradually  to  zero,  the 
point  tracing  the  diagram  line  would  have  retraced  the 
line  to  A,  and  the  test  piece  would  have  recovered  its 
original  dimensions. 

But  if  the  relief  of  stress  had  been  delayed  until  some 
point  beyond  the  elastic  limit  R  was  reached,  the  tracing 
point  would  return  to  the  X  axis  over  the  line  R  T  nearly 
parallel  to  B\A.  During  this  return  a  certain  portion  of 
the  elongation  (elastic  deformation)  ST  would  disappear, 
while  another  portion,  TA,  would  remain  as  permanent  set. 

The  physical  properties  that  appear  on  the  s.-d.  diagram 
are: 

1.  Strength  at  elastic  limit. 

2.  Strength,  ultimate. 

3.  Ductility. 

4.  Elasticity. 

5.  Stiffness. 

6.  Resilience,  elastic. 

7.  Resilience,  ultimate. 

1.  The  strength  at  elastic  limit  is  proportional  to  the 
ordinate  AB,  whose  value  may  be  read  in  pounds  per 
square  inch. 

2.  The  ultimate  strength  is  proportional  to  the  maxi- 
mum ordinate  DDi,  whose  value  in  pounds  per  square 
inch  may  also  be  read. 

*  For  an  explanation  of  the  probable  reason  for  the  horizontal 
part  of  the  diagram  line,  see  page  155. 


100  MATERIALS  OF    MACHINES 

3.  A  material  is  ductile  that  stretches  under  an  increas- 
ing tensile  force  before  it  breaks.     Ductility  is  therefore 
proportional  to  the  amount  of  stretching.     Hence  it  is 
proportional  to  the  length  of  the  s.-d.  diagram  on  the  axis 
of  X.     If  s.-d.  diagrams  of  different  materials  are  plotted 
with  the  same  scale  for  values  of  e,  the  relation  of  their 
ductilities  can  be  found  by  comparison  of  the  lengths  of 
the  diagrams  on  the  X  axis.     The  value  of  e  at  rupture 
might  be  taken  as  a  measure  of  ductility;  but  in  engineer- 
ing test  practice  it  is  customary  to  measure  the  elonga- 
tion of  the  tested  section  of  the  test  piece  after  rupture, 
and  to  compare  this  with  the  original  length  of  the  tested 
section.     Ductility  is  thus  expressed  as  per  cent  elonga- 
tion in  I  inches,  I  being  original  length  of  tested  section. 
It  is  necessary  to  specify  the   original  length  because, 
from  the  maximum  stress  until  rupture,  the  elongation  is 
localized  and  is  not  shared  by  all  one-inch  sections  alike, 
hence  the  average  elongation  depends  on  how  many  of 
the  unnecked  sections  are  included  with  the  necked  section 
in  finding  the  average.     For  example,  the  average  elonga- 
tion derived  from  the  necked  section  taken  with  two  other 
sections  would  be  greater  than  the  value  derived  from  the 
necked  section  with  any  greater  number  of  other  sections.* 

4.  Elasticity.  —  When  the  initial  part  of  the  s.-d.  dia- 
gram is  a  straight  line,  it  is  an  indication  that  the  material 
is    practically    perfectly    elastic    for    the    corresponding 
range.     Thus  in  diagram  II,  Fig.  17,  if  the  material  is 
perfectly  elastic  for  the  range  corresponding  to  AB,  it 
could  be  stressed  within  these  limits  an  indefinite  number 

*  The  value  of  e  for  the  point  E  in  diagram  I  is  derived  from 
the  permanent  elongation  after  rupture;  whereas  for  all  previous 
points  of  the  diagram  the  elastic  elongation  is  included  with  the 
permanent  elongation  for  the  computation.  The  error  due  to  this, 
however,  is  small  enough  to  be  negligible. 


TESTING  MATERIALS 


101 


of  times  and  each  time  it  would  return  to  its  original  di- 
mensions.    But  in  an  s.-d.  diagram  like  ACB,  Fig.  18  — 
which  is  a  record  of  a  test  of  cast  iron  —  there  is  no  elastic 


5 

< 
o: 

1 

Q 

go 

H  ~ 

<  H 

O  <f> 

o  o 

-1    U. 

LU  O 

m\ 

en 

\ 

LU 

\ 

H 

QTOft* 

^ 

^ 

CTOO*  &   06 

^ 

s^ 

f  TOO*  5i 

\ 

XX 

prm-  §   2 

X 

Xs 

M     fe 

N 

\X 

^x 

2100   M 

\ 

^ 

< 

\ 

XN 

Rnnn*  2 

\ 

x 

^s 

X 

S. 

V 

x> 

N 

xe 

X 

N 

^  i 

yooo  § 
T-nnn*  rt 

K 

"N 

ij 

Pfinn* 

^> 

• 
*  —   i 

znfifi* 

^\r 

TfWli* 

i 

\ 

o 

(J  =  qouj  oj^nbg  aod  spuno j  ut  ssaa^s 

range  and  even  small  stress  produces  set.  In  this  test 
stress  was  relieved  at  C  and  the  diagram  line  traced  the 
path  CDE,  showing  a  set  at  E  *  proportional  to  EG.  Then 

*  Stress  was  not  reduced  to  zero  because  of  difficulties  that 
would  result  in  manipulation  of  the  apparatus  for  measuring  elon- 
gation. 


102  MATERIALS  OF   MACHINES 

stress  was  again  increased  until  rupture  occurred  and  the 
diagram  line  followed  the  path  EFCB.  It  is  evident  that 
upon  relief  and  reapplication  of  stress  the  line  is  not 
straight  from  C  to  E  and  again  from  E  to  C.  The  disap- 
pearance of  elastic  deformation  occurs  at  practically  a 
uniform  rate  from  C  to  D  —  or  CD  is  a  straight  line  - 
and  from  some  point  D  the  rate  of  disappearance  of  elastic 
deformation  increases  to  E;  that  is,  DE  is  curved.  The 
line  of  reapplication  of  stress  EFC  similarly  is  straight 
from  E  to  some  point  F,  while  FC  is  a  curve  ending  at 
the  starting  point  C.* 

It  is  obvious  that  the  first  application  of  stress  has 
produced  an  artificial  elastic  limit  F,  together  with  a  cor- 
responding elastic  range  EF,  and  the  stressed  material 
may  be  considered  as  artificially  elastic  through  this 
range. 

The  width  of  the  loop  CDEFC  is  only  slightly  affected 
by  the  time  occupied  in  changing  stress  from  C  to  E  and 
back  again  to  C;  and  hence  the  loop  phenomenon  must  be 
a  function  of  some  quality  of  the  material.  Very  careful 
measurements  lead  to  the  conclusion  that  the  same  phe- 
nomenon occurs  with  ductile  material,  but  the  corre- 
sponding values  are  too  small  to  be  detected  by  the 
measuring  instruments  of  ordinary  testing  practice  of 
engineering. 

5.  Stiffness  of  a  material  is  a  function  of  the  amount  of 
yielding  under  given  force.  It  is  measured  by  the  ratio 
within  the  elastic  limit  of  unit  stress  to  relative  elonga- 
tion =  -,  and  this  ratio  is  called  the  modulus  of  elas- 

*  If  relief  of  stress  had  been  complete  the  curve  DE  would  have 
continued  to  the  X  axis  and  would  have  returned  at  the  left  of  EF, 
but  a  final  curve  would  have  brought  the  line  to  the  same  destination 
C  as  in  starting  from  E.  Thus  the  loop  would  be  made  wider. 


TESTING   MATERIALS  103 

ticity  for  tension,  Et.*  Since  for  the  material  represented 
by  Fig.  17  p  is  proportional  to  e  within  the  elastic  limit,  it 
follows  that  any  corresponding  values  of  p  and  e  between 
A  and  B  —  those  at  N,  for  example  —  may  be  taken  to 
compute  the  value  of  Et.  The  variation  in  stiffness  has 
much  narrower  limits  than  tensile  strength;  thus,  low- 
carbon  open-hearth  steel  may  have  a  tensile  strength  of 
about  60,000  pounds  per  square  inch,  while  high-carbon 
crucible  steel  may  have  a  tensile  strength  above  100,000 
pounds  per  square  inch,  a  variation  of  100  per  cent,  proba- 
bly; yet  the  values  of  Et  corresponding  may  be  28,000,000 
and  32,000,000,  a  maximum  increase  of  about  14  per  cent. 
Practically  all  values  of  Et  for  steel  —  of  whatever  grade  — 
fall  within  the  limits  just  given. 

In  the  tension  s.-d.  diagram  for  cast  iron,  Fig.  18,  ABC, 
the  stress-deformation  line  is  curved  continuously  from 

start  to  rupture,  and  hence  the  value  of  Et  =  -  varies 

continuously  and  the  original  material  cannot  be  said  to 
have  a  definite  modulus  of  elasticity.  But  since  the 
material  by  relief  and  reapplication  of  stress  from  some 
point  —  as  for  example,  C  —  is  given  an  artificial  elastic 
range,  it  follows  that  the  stressed  material  would  have  an 
artificial  value  of  Et,  measuring  stiffness,  corresponding  to 

nr\ 

—  for  any  point  in  the  straight  line  EF.     This  value  of 

artificial  modulus  of  elasticity,  Et,  for  cast  iron  probably 
averages  about  15,000,000  pounds  per  square  inch. 

*  The  modulus  of  elasticity  Et  is  qualitatively  the  same  as  unit 
stress  p]   that  is,  it  is  a  value  expressed  in  pounds  per  square  inch; 

for,  Et  =  -  =  p  -  -  in  which   -,  being  a  ratio  of  linear  dimensions, 
e  X  X 

is  an  abstract  quantity;  hence  Et  equal  to  pounds  per  square  inch 
multiplied  by  an  abstract  number  must  also  equal  some  value 
expressed  in  the  same  units  as  p. 


104  MATERIALS  OF  MACHINES 

Resilience  is  the  name  given  to  the  work  done  within 
a  material  while  its  form  is  changed  by  external  forces. 
It  is  therefore  the  summation  of  the  product  of  all 
stresses  produced,  multiplied  by  their  yielding.  But 
this  summation  is  equal  to  the  work  done  by  external 
forces  in  producing  change  of  form;  hence  this  work 
is  a  measure  of  resilience. 

In  s.-d.  diagram  II,  Fig.  17,  work  is  done  by  the  unit 
force  which  is  initially  zero  and  which  increases  uniformly 
to  the  elastic  limit.  Let  unit  force  at  the  elastic  limit  be 
represented  by  p\',  then  the  average  unit  force  up  to  the 

elastic  limit  equals  ^ .     This  unit  force  acting  upon  one 

Z 

square  inch  section  area  has  caused  an  elongation  equal 
to  €  in  each  one-inch  section  of  length  of  the  test  piece, 

and  hence  ^  e  equals  the  work  done  on  one  cubic  inch  of 

2 

the  test  piece  in  bringing  it  to  the  elastic  limit.  This 
value  is  sometimes  called  the  modulus  of  resilience  of -the 
material  and  is  represented  by  Ut. 

The  total  elastic  resilience,  equal  to  the  work  done 
on  the  tested  portion  of  the  test  piece  up  to  the  elastic 

limit,  is  equal  to  the  total  force  P  =  ^  F,  multiplied  by 
the  total  elongation  =  X  =  el  or  is  equal  to  PX  =  ^  eFL 

£t 

This  value  is  proportional  to  the  area  of  the  triangle  ABiG 
and  is  equal  to  the  modulus  of  resilience  multiplied  by 
Fl,  the  volume  of  the  tested  portion  of  the  test  piece. 

The  ultimate  resilience  is  the  work  done  in  breaking 
the  test  piece.  Consider  s.-d.  diagram  I,  Fig.  17.  The 
mean  height  of  the  diagram  Ym  measures  the  mean  unit 
force  acting  throughout  the  test;  while  AEi  —  the  final 
value  of  €  —  measures  the  average  elongation  of  each 
one-inch  section  of  the  tested  piece.  Hence  the  product 


TESTING  MATERIALS  105 

of  Ym  and  AEi  —  both  expressed  in  inches  —  gives  an 
area  in  square  inches  that  is  proportional  to  the  ultimate 
resilience  per  cubic  inch  of  the  material.  This  value  is 
obviously  equal  to  the  area  of  the  diagram.  Numeri- 
cally this  value  in  inch  pounds  per  cubic  inch  equals  the 
average  value  of  p  in  pounds  per  square  inch,  multiplied 
by  the  final  value  of  e  in  inches  per  inch  of  test  piece. 

A  machine  member  may  be  subjected  to  shock  in  use 
and  it  is  necessary  to  know  the  shock-resisting  capacity 
of  materials  of  machines. 

If  a  tensile  shock  were  delivered  to  the  piece  whose 
test  is  recorded  in  Fig.  17,  and  if  the  energy  of  the  shock 
were  equal  to  the  energy  represented  by  the  area  ABiG, 
it  follows  that  the  shock  would  stress  the  piece  to  its 
elastic  limit.  Hence  the  area  ABiG  is  proportional  to 
the  shock-resisting  capacity  of  the  material  at  the  elastic 
limit.  If  s.-d.  diagrams  of  different  materials  were  plotted 
on  the  same  scales,  areas  of  triangles  under  the  elastic 
limit  could  be  compared  to  determine  relative  moduli 
of  resilience  or  elastic  shock-resisting  capacity. 

Similarly,  areas  under  the  complete  s.-d.  diagrams  meas- 
ure the  capacity  of  the  materials  to  resist  rupture  by  shock, 
and  comparison  of  these  areas  —  in  diagrams  on  the  same 
scales  —  gives  relative  ultimate  shock-resisting  capacities. 

In  Fig.  19  diagram  I  is  a  reproduction  on  an  enlarged 
scale  of  the  initial  part  of  the  s.-d.  diagram  of  mild  steel 
given  in  Fig.  17.  Diagram  II,  Fig.  19,  is  the  s.-d.  diagram 
of  stressed  cast  iron  like  that  represented  in  Fig.  18.  The 
elastic  limit  of  the  mild  steel  is  at  B,  while  the  artificial 
elastic  limit  of  the  cast  iron  is  at  BI.  The  modulus  of 
resilience  of  the  steel  is  proportional  to  the  area  ABC, 
while  that  of  the  cast  iron  is  proportional  to  the  area 
ABiCi.  Obviously  the  steel  has  greater  elastic  resilience 
than  the  stressed  cast  iron. 


106 


MATERIALS  OF   MACHINES 


oouuu 
33000 
30000 
,g27000 
£24000 

^21  000 

I 
$18000 
3 
(S  15  000 

.S 

§12000 
^9000 
6000 
3000 

A 

I 

iai 

a-£ 

111. 

T 

/ 

/f 

/ 

1 

k 

N 

/ 

s* 

** 

E 

Di 

Ig 

•ui 

tt] 

I 

1 

/ 

/ 

1 

x 

/ 

BI 

1 

/ 

/ 

1 

/ 

/ 

M 

1 

/ 

/ 

/ 

y 

7 

/ 

/ 

K 

C 

Ci 

G 

Hcv* 
SoooSoSooSSSSSoSSSoo 

Belative  Elongation  in  inches  per  inch  original  length 

FIG.  19. 


All  of  the  elastic  properties  of  mild  steel  and  stressed 
cast  iron  can  now  be  compared  by  reference  to  Fig.  19. 


Strength  at  elastic  limit  of  mild  steel          _  BC 
Strength  at  elastic  limit  of  stressed  cast  iron     Bid 

for  this  case. 


=  1.86 


nr\ 

Since  stiffness  is  proportional  to  -  within  the  elastic 
limit,  and  since  this  ratio  is  constant  within  the  elastic 


TESTING   MATERIALS  107 

limit,  it  follows  that  any  value  of  e  less  than  AC  may  be 
chosen  for  the  comparison;  as  for  example,  AK. 

KN 

Stiffness  of  mild  steel         =  AK    =  KN  =  .  g. 
Stiffness  of  stressed  cast  iron  ~KM~  KM  ~ 

AK 
for  this  case. 

The  relative  elongation  at  elastic  limit  is  measured  by 

,.    AC 
the  ratio  -r~- • 


Modulus  of  resilience  of  mild  steel 

ron 

or^Q    A  TZC1 

1.8 


Modulus  of  resilience  of  stressed  cast  iron 

area  ABC 


area  ABiC 

for  this  case.  This  measures  the  comparative  shock- 
resisting  capacity  at  the  elastic  limit  of  the  two  materials 
tested. 

The  ultimate  resilience  of  the  stressed  cast  iron  is  pro- 
portional to  the  area  AEG]  while  the  ultimate  resilience 
of  the  steel  is  proportional  to  the  total  area  under  the 
steel  diagram  which  extends  far  beyond  the  limits  of 
Fig.  19  (see  diagram  I,  Fig.  17). 

Compression.  —  In  s.-d.  diagrams  of  compressive  tests 
unit  stress  is  plotted  downward  and  relative  deformation 
is  plotted  toward  the  left.  Fig.  20  shows  diagrams  of 
mild  and  high-carbon  steel  and  of  cast  iron  both  in  tension 
and  in  compression.  Even  if  the  strength  of  a  given 
ductile  material  were  the  same  in  tension  and  compression, 
the  compression  diagram  would  have  greater  values  of 
unit  stress  for  the  following  reason: 

In  a  tension  test,  elongation  is  accompanied  by  reduc- 


108 


MATERIALS  OF   MACHINES 


tpui  wribja    spnnoj  uoissajduioQ  tpui  airmbg  aad  spunoj 


TESTING   MATERIALS  109 

tion  of  section  area,  since  the  density  remains  unchanged. 
The  true  unit  stress  at  any  point  of  the  test  would  be 
total  tensile  force  divided  by  the  corresponding  section 
area;  but  it  has  been  decided  for  engineering  tests  to  use 
the  original  section  area  as  a  divisor  throughout  the  test 
and  hence  all  values  of  unit  stress  in  tension  are  really 
too  small.  On  the  other  hand,  in  a  compression  test  the 
section  area  increases  with  reduction  in  length  and 
hence  all  values  of  unit  stress  in  compression  figured  on 
the  original  section  area  are  really  too  large.  In  the 
comparison  of  tension  tests  with  each  other  this  would 
not  lead  to  error,  nor  would  it  lead  to  error  in  case  of 
comparison  of  compression  tests  with  each  other;  com- 
parisons of  tension  with  compression  tests  are  probably 
unnecessary.  This  discrepancy  in  the  s.-d.  diagrams  is  of 
no  importance  within  the  elastic  limit,  since  the  change 
in  section  area  up  to  that  limit  is  negligible. 

A  ductile  material  like  mild  steel  fails  under  compressive 
force  either  by  splitting  parallel  to  its  axis,  or  by  flatten- 
ing out  with  a  continually  increasing  section  area,  hence 
it  is  impossible  to  locate  a  definite  breaking  point. 

A  brittle  material  fails  in  compression  by  shearing  on 
planes  at  about  45  degrees  to  the  axis  of  the  test  piece. 

S.-d.  diagrams  may  also  be  plotted  from  the  data  of 
tests  in  which  the  external  forces  produce  torsional  or 
transverse  stress  as  well  as  from  the  data  of  tensile  and 
compressive  tests. 


CHAPTER  VII 

THE  EQUILIBRIUM  DIAGRAM  OF  IRON 
AND  CARBON 

CERTAIN  pure  chemical  elements  occur  in  two  or  more 
so-called  allotropic  forms  whose  physical  properties  are 
quite  different.  Thus,  diamond,  graphite  and  charcoal 
are  allotropic  forms  of  carbon;  oxygen  occurs  in  two 
allotropic  forms,  02  and  03  (ozone).  In  general,  change 
from  one  allotropic  form  to  another  is  accompanied  by 
absorption  or  release  of  energy.  The  change  may  be  due 
to  rearrangement  of  molecules  or  of  atoms,  or  to  some 
other  unexplained  cause. 

Iron  has  three  allotropic  forms  called  alpha  (a)  iron, 
beta  (|S)  iron  and  gamma  (7)  iron.  Each  form  is  stable  — 
that  is,  it  resists  change  into  the  other  forms  —  within 
certain  temperature  limits.  This  is  shown  in  Fig.  21. 
Temperature  values  in  degrees  Fahrenheit  are  laid  off 
vertically  upward.  Horizontal  spaces  represent  conven- 
tionally the  time  of  heating  or  cooling.  Between  any 
attainable  lower  temperature  and  1418°  F.  iron  takes  the 
form  of  solid  a  iron;  at  1418°  F.  it  changes  to  solid  ft 
iron  and  holds  this  form  until  1660°  F.  is  reached,  where 
it  changes  to  solid  7  iron  which  melts  at  2786°  F. 

The  diagram  represents  heating,  but  the  changes  begin 
at  the  same  temperatures  whether  the  temperature  change 
is  upward  or  downward;  whether  the  iron  is  heated  or 
cooled.  Alpha  iron  is  soft  and  ductile  with  tensile  unit 
strength  of  about  40,000  pounds  at  air  temperature.  It 

110 


EQUILIBRIUM  OF  IRON  AND  CARBON          111 

is  very  magnetic,  but  loses  this  property  entirely  on  chang- 
ing into  |8  iron.  The  crystal  structure  of  «  iron  is  like  that 
of  0  iron,  but  is  quite  different  from  that  of  7  i 


Liqui< 

1  Iron 

/ 

278 

/ 

2600 

Soli( 

• 

r'    ll- 

:>n  / 

OOflfl 

/ 

/ 

2UUU 

/ 

7 

1600 
1400 
1200 
1000 
800 
600 
400 

/ 

. 

418%. 

/ 

S 

alld  (i 

Eron 

/ 

iolld  ( 

'  Iron 

/ 

/ 

/ 

1 

/ 

0 

60°/ 

HEATING  CURVE  OF  PURE  IRON 
FIG.  21. 


certain  small  amount  of  heat  becomes  latent  during  the 
change  of  a  to  (3  iron,  and  a  larger  amount  becomes  latent 
during  the  0  to  7  change.  These  and  other  facts  show 


112  MATERIALS  OF  MACHINES 

that  a,  j8  and  7  iron  are  allotropic  forms  with  distinct 
physical  differences. 

Liquid  iron  will  take  freely  into  liquid  solution  many 
metallic  elements,  such  as  manganese,  nickel,  cobalt, 
chromium,  tungsten,  vanadium  and  copper,  as  well  as 
non-metallic  elements,  such  as  silicon,  carbon,  phosphorus 
and  nitrogen;  certain  quantities  of  these  remain  in  solid 
solution  when  the  iron  solidifies  in  the  7  form.  When 
the  cooling  progresses  into  the  /3-iron  field,  though  the 
metallic  elements  are  probably  held  in  solid  solution, 
the  non-metallic  elements  seem  not  to  be  retained.  With 
further  cooling  into  the  a-iron  field,  moderate  amounts  of 
the  metallic  elements  are  held  in  solid  solution,  while  the 
non-metallic  elements  are  held  only  slightly  or  not  at  all. 

Carbon  is  the  most  important  element  whose  presence 
exerts  a  modifying  influence  upon  iron.  It  may  be  asso- 
ciated with  solid  iron  in  solution,  in  chemical  combination 
or  in  mechanical  mixture.  The  phenomena  accompanying 
changes  of  temperature  of  associated  iron  and  carbon 
can  be  best  explained  by  use  of  the  so-called  equilibrium 
diagram.  Such  a  diagram  is  given  in  Fig.  22;  it  is  drawn 
only  with  approximate  accuracy.  Temperatures  are 
laid  off  vertically  upward  on  the  axis  of  Y,  and  percentages 
of  carbon  present  with  the  iron  are  laid  off  horizontally 
toward  the  right  on  the  axis  of  X.  At  A  is  the  solidifica- 
tion temperature  of  pure  iron  and  solidification  becomes 
complete  at  this  temperature;  but  as  carbon  is  added  to 
the  iron  the  solidification  begins  at  lower  temperatures,  as 
shown  by  the  line  AD.  Moreover,  while  pure  iron  be- 
comes completely  solid  at  the  temperature  A,  associated 
iron  and  carbon  begin  to  solidify  at  temperatures  defined 
by  AD,  but  solidification  does  not  become  complete 
until  some  lower  temperature,  defined  by  AB,  is  reached. 
For  illustration,  the  point  h  represents  liquid  iron  contain- 


EQUILIBRIUM   OF  IRON  AND  CARBON          113 


3000 
2800 
2600 
2400 

2200 

2050 
2000 

1800 

1660 
1600 

1400' 

e        /   Q     h 

N 

3                              n 

A 

1 
| 

E 

1 

1 

m/ 

/ 

]s7h 

Irli 

^r^2 

LIQ 

JID  S 

3LUTI 

)N 

/  L 

quid 

pi 

V      | 

~^^* 

I 

N£— 

-=y-(G) 

^s^.^ 

^ 

li 
/+ 

^+G  ( 
(line 

table) 
ableL. 

F 

fX 

+  Li 

uid 

f 

I 
1 

\ 

B 

h 

^ 

-v^D 

/ 

• 

I 
1 

1 
1 

/ 

TVc: 

I/ 

7 

7(C) 

*-G  (st 
3  CCu 

able) 
nstable 

S{ 

I 

p//H 

R 

7 

C)+Fe 

,C    (sable) 

^ 

+  G  (unstable) 

2 

1200 
1000 
800 
600 
400 
200 
0 

M   ^ 

o 

a 

,7(C 

*-  — 

—  j- 

x  +  Fe 

C    (8t£ 

ble)  + 

G    («n 

table) 

0.9 

Ni 

0.5       I       J,5      2 

2.5       3       3.5       4        4.5        5        5.5       6 
Percent  of  Carbon 

FIG.  22. 

ing  about  1J  per  cent  of  carbon  at  a  temperature  of 
3000°  F.  During  cooling  the  point  moves  from  h  vertically 
downward,  and  when  it  reaches  d  on  the  line  AD  solidifica- 
tion begins,  but  is  not  completed  until  the  point  moving 
vertically  —  with  falling  temperature  —  reaches  b  on  the 
line  AB. 


114  MATERIALS  OF  MACHINES 

The  first  crystals  that  form  do  not  contain  the  propor- 
tions corresponding  to  d,  but  a  smaller  amount  of  carbon; 
in  fact  the  composition  is  indicated  by  the  point  o2,  where 
a  horizontal  through  d  cuts  AB.  As  d  descends,  the  com- 
position of  the  forming  crystals  is  continuously  indicated 
by  the  intersection  with  AB  of  the  horizontal  through 
the  moving  point;  thus  with  the  point  at  di  the  composi- 
tion of  the  forming  crystals  is  indicated  by  61 ;  at  the 
same  time  the  composition  of  the  residual  liquid,  which 
grows  constantly  richer  in  carbon,  is  indicated  by  d%  where 
the  horizontal  cuts  AD.  Thus  crystals  form  continuously 
with  increasing  carbon  content  and  this  results  in  formation 
of  crystal  groups  with  cores  low  in  carbon  surrounded  by 
layers  of  crystals  having  increasing  carbon  content. 

Now,  since  the  first-formed  crystals  have  low  carbon 
and  since  the  average  carbon  content  cannot  change,  it 
follows  that  the  later-formed  crystals  must  be  high  in 
carbon,  and  hence  the  outermost  layers  of  the  crystal 
groups  must  have  carbon  content  much  above  the  average 
value,  and  hence  the  point  62  on  AB,  instead  of  stopping 
at  6,  continues  toward  B  to  some  point  that  holds  the 
average  at  6. 

Obviously  the  solid  cannot  be  homogeneous  unless  it 
is  made  so  during  or  after  solidification  by  redistribution 
of  carbon  by  diffusion.  Diffusion  through  the  solid  is  a 
slow  process  and  hence  cooling  must  be  very  slow  through- 
out the  solidification  range  and  afterwards  if  a  homo- 
geneous solid  is  to  result. 

In  case  of  steel  ingots,  which  are  "stripped"  and 
transferred  to  "  soaking-pits  "  for  slow  cooling  and  heat 
equalization  while  the  ingot  interior  is  still  liquid,  un- 
doubtedly there  is  time  for  diffusion  of  carbon  to  pro- 
duce uniform  distribution.  But  in  steel  castings  the 
cooling  is  too  quick  to  permit  this  result  and  there  is, 


EQUILIBRIUM  OF  IRON  AND  CARBON          115 

as  micro-photographs  show,  a  lack  of  uniformity  of  carbon 
content  in  the  crystals. 

While  the  point  is  above  AD  the  liquid  iron  holds  the 
1 J  per  cent  carbon  in  liquid  solution;  between  AD  and  AB 
the  carbon  is  partly  in  liquid  and  partly  in  solid  solution, 
and  below  AB  the  mass  is  solid,  the  carbon  being  in  solid 
solution  in  the  iron.  The  results  of  cooling  will  be  similar 
for  any  amount  of  carbon  up  to  about  2.1  per  cent,  or 
with  the  cooling  point  located  anywhere  above  AD  and  at 
the  left  of  NNi.  Iron  will  hold  carbon  in  liquid  solution 
in  amount  according  to  temperature,  as  shown  by  the 
limiting  line  DE.  Thus  at  2600°  F.  iron  will  hold  5.4  per 
cent  carbon  corresponding  to  the  point  M]  at  2800°  F. 
the  liquid  solution  is  saturated  with  5.75  per  cent  carbon; 
see  point  E.  But  2.1  per  cent  seems  to  be  about  the  limit 
of  carbon  in  solid  solution  in  iron.  Therefore,  if  the 
molten  mass  contains  more  than  2.1  per  cent  carbon  the 
excess  must  separate  on  solidification.  This  may  be 
illustrated  on  the  diagram.  The  point  j  represents 
3  per  cent  carbon  in  liquid  solution  in  97  per  cent  iron  at 
3000°  F.  As  this  liquid  cools  the  point  moves  vertically 
downward  and  when  ji,  on  AD,  is  reached,  solidification 
begins;  but  the  solid  crystals  first  formed  can  only  contain 
the  amount  indicated  by  63,  about  1.3  per  cent  carbon, 
and  hence  it  must  have  given  up  the  residue  of  its  original 
3  per  cent  which  is  added  to  the  residual  liquid  whereby 
that  liquid  is  enriched  in  carbon.  Thus  the  cooling  point 
representing  tiio  :  imposition  of  the  residual  liquid 
moves  toward  the  ngL*::  in  fact  it  moves  along  the  line 
AD.  Meanwhile  the  point  representing  the  composition 
of  the  crystals  moves  along  the  line  AB  from  63  toward  B 
and  the  residual  liquid  grows  richer  in  carbon  until  the 
cooling  point  that  started  from  ji  reaches  j2 ;  the  residual 
liquid  composition  corresponds  to  D  and  the  mass  solidi- 


116  MATERIALS  OF  MACHINES 

fies  without  further  drop  in  temperature.  The  product 
of  this  final  solution  is  y  (C),  associated  with  whatever 
is  crowded  out  during  the  entire  change  from  liquid  to 
solid.  Since,  by  assumption,  the  other  substance  present 
is  carbon,  it  follows  that  only  carbon  or  compounds  of  car- 
bon and  iron  could  be  associated  with  the  y  (Q .  The 
solidified  mass  really  does  consist  of  intimately  associated 
crystals  of  y  (C),  carbon  as  graphite,  G,  and  iron  carbide, 
Fe3C. 

But  the  starting  point  of  cooling  might  have  been  at 
some  point  at  the  right  of  nD,  for  example,  I,  representing 
a  liquid  solution  of  5  per  cent  carbon  in  iron.  The  point 
Z,  moving  vertically  downward  during  cooling,  reaches  the 
line  DE  where  the  liquid  iron  is  saturated  with  carbon 
and  where  further  fall  in  temperature  results  in  separa- 
tion of  solid  crystals,  probably  Fe3C  *,  which  may  change 
in  part  to  graphite  in  the  effort  to  establish  equilibrium. 
The  remaining  liquid  is  thus  impoverished  in  carbon  and 
hence  the  point  representing  composition  of  residual 
liquid  moves  toward  the  left;  in  fact  it  follows  the  line 
ED  to  D,  and  during  this  progress  Fe3C  separates  continu- 
ously. At  D  the  residual  liquid  contains  4.5  per  cent 
carbon  in  liquid  solution,  and  during  final  solidification 
at  constant  temperature  the  excess  of  carbon  over  2.1 
per  cent  is  crowded  out  as  Fe3C  and  graphite.  The 
product  of  solidification  at  D  from  initial  condition  j  and 
at  I  are  similar,  but  of  course  with  different  proportions 
of  7(C)"andG  +  Fe8C. 

With  n  as  the  starting  point  of  cooling  the  liquid  con- 
sists of  a  4.5  per  cent  solution  of  carbon  in  iron  and  the 
cooling  line  cuts  AD  and  ED  at  their  intersection  D. 

*  It  seems  to  be  accepted  as  a  fact  that  when  two  forms  may 
separate  from  a  solution  the  form  that  is  unstable  under  the  con- 
ditions separates  first. 


EQUILIBRIUM  OF  IRON  AND  CARBON        .117 

Hence  no  preliminary  separation  of  7  (C)  occurs  as  with 
the  line  of  cooling  at  the  left  of  nD,  nor  preliminary  sepa- 
ration of  FesC  and  graphite  as  with  the  line  of  cooling  at 
the  right  of  nD,  but  the  solution  remains  constant  until 
D  is  reached,  and'  solidification  begins  and  ends  at  that 
temperature.  Before  solidification  the  mass  is  all  liquid 
solution  with  4.5  per  cent  carbon;  after  solidification 
the  iron  holds  2.1  per  cent  carbon  in  solid  solution,  but 
the  excess  of  carbon  above  2.1  per  cent  separates  as  ce- 
mentite  (unstable)  or  graphite  (stable).  Hence  the  solid 
product  of  cooling  from  n  consists  of  intimately  mixed 
crystals  of  7  (C),  Fe3C  and  G  with  the  total  carbon  present 
equal  to  4.5  per  cent.  This  solid  is  called  "eutectic," 
and  D  is  the  "  eutectic  point." 

With  cooling  from  j  the  solid  product  consists  of  eutectic 
formed  at  D  mixed  with  excess  of  7  (C)  with  varying 
carbon  content  formed  during  temperature  change  from 
ji  to  J2>  With  cooling  from  I  the  solid  product  consists 
of  eutectic  formed  at  D  mixed  with  excess  of  G  and  FesC 
formed  during  temperature  change  from  li  to  12. 

Certain  changes  in  composition  of  associated  iron  and 
carbon  also  occur  as  temperature  falls  after  complete 
solidification. 

The  lines  KNO,  BO,  LN  and  MO  (Fig.  22)  have  been 
located  on  the  diagram  by  careful  experiments.  At  K 
is  the  temperature  of  interchange  between  the  j8  and  7 
forms  of  pure  iron.  The  sloping  line  KN  indicates  the 
drop  in  temperature  of  (3,  7  interchange  due  to  increasing 
proportion  of  carbon.  At  L  is  1420°  F.,  the  temperature 
of  interchange  between  a  and  ]8  forms  of  pure  iron,  and 
between  L  and  N  any  free  iron  in  the  mass  would  inter- 
change between  0  and  a  at  the  constant  temperature  LN. 
The  sloping  line  NO  indicates  the  drop  in  temperature  of 
a,  7  interchange  due  to  increasing  proportion  of  carbon. 


118  MATERIALS  OF  MACHINES 

The  line  BO  defines  the  lowest  temperatures  at  which 
7  (C)  solid  solutions  can  exist  without  separation  of 
cementite  or  graphite,  or  it  is  the  line  of  solid  saturation. 

First  illustration. —  The  point  e  represents  a  solution 
of  0.2  per  cent  carbon  in  liquid  iron  at  3000°  F.  As  this 
point  moves  vertically  downward  on  the  line  ee\  during 
slow  cooling,  it  passes  the  line  AD  where  solidification 
begins,  and  the  line  AB  where  solidification  ends.  The 
point  continues  to  move  vertically  downward,  the  cooling 
material  remaining  7  (C)  and  with  slow  cooling  probably 
becoming  homogeneous  by  diffusion,  until  the  line  KN 
is  reached  at  e\\  at  this  point  some  of  the  iron  changes 
from  the  7  (C)  to  the  ft  form,  and  since  ft  iron  cannot 
hold  carbon  in  solution,  the  7  (C)  that  is  thus  left  is 
impoverished  in  iron;  hence  the  cooling  point  moves  in 
the  direction  of  increased  carbon  toward  the  right;  in  fact 
it  moves  along  the  line  KN  until  the  point  N  is  reached; 
at  this  temperature,  1420°  F.,  the  ft  iron  that  has  sepa- 
rated out  as  pure  iron  changes  —  as  any  pure  iron  would 
at  that  temperature  —  into  the  a  form,  while  more  iron 
separates  from  the  7  (C)  as  a  iron,  causing  further  iron 
impoverishment  of  the  remaining  7  (C) ;  then  the  cooling 
point  moves  along  the  new  slope,  NO,  until  the  point  0  is 
reached  at  the  lowest  temperature  at  which  iron  can  exist 
in  the  7  form,  and  the  residue  of  7  (C)  changes  into  a 
eutectic  mixture  called  pearlite,  consisting  of  a  iron  and 
Fe3C. 

Second  illustration.  —  Let  /  be  the  starting  point  of 
cooling  with  0.7  per  cent  carbon.  As  before,  the  cooling 
point  passes  the  lines  AD  and  AB,  where  solidification  of 
7  (C)  is  completed,  and  arrives  at  /i  on  the  line  NO,  where 
7  iron  changes  into  a  iron  (possibly  passing  through  the 
intermediate  ft  form)  impoverishing  the  remaining  7  (C) 
in  iron  and  causing  the  point  to  move  along  the  line  NO 


EQUILIBRIUM   OF  IRON  AND  CARBON          119 

to  the  point  0  where  the  eutectic  pearlite  is  formed  as  in 
the  first  illustration. 

Third  illustration.  —  With  g  as  the  starting  point  of 
cooling  the  carbon  equals  0.9  per  cent,  which  is  just  the 
proportion  corresponding  to  the  eutectic;  the  point  goes 
vertically  downward  passing  AD  and  AB,  the  range  of 
solidification,  and  continuing  to  0  where,  since  this  is 
the  eutectic  point,  the  7  (C)  changes  directly  into  the 
eutectic  pearlite. 

Fourth  illustration.  —  With  h  as  the  starting  point  of 
cooling,  the  carbon  equals  1.25  per  cent  and  the  point 
moves  through  the  solidification  range  and  finally  reaches 
hi  on  the  line  BO.  Here  the  condition  is  that  of  a  solid 
saturated  solution  of  carbon  in  iron  and  hence  further 
reduction  in  temperature  results  in  separation  of  Fe3C. 
The  result  is  that  the  remaining  7  (C)  is  impoverished 
in  carbon  and  hence  the  point  must  move  to  the  left; 
it  really  does  move  along  the  line  BO  and  finally  reaches  0, 
where  the  7  (C)  changes  into  the  eutectic  pearlite  as  before. 

If  the  cooling  point  starts  at  the  left  of  g,  iron  separates 
from  the  7  (C)  along  the  line  KNO  to  leave  the  eutectic 
proportion  at  0,  and  the  resulting  mass  consists  of  crys- 
tals of  a  iron  intermixed  with  eutectic  pearlite. 

With  g  as  the  starting  point  of  cooling  all  7  (C)  changes 
directly  into  eutectic  and  hence  the  resulting  mass  will 
consist  of  the  eutectic  alone. 

With  the  starting  point  of  cooling  at  the  right  of  g, 
Fe3C  separates  from  the  7  (C)  along  the  line  BO  to  leave 
the  eutectic  proportion  at  0,  and  the  resulting  mass  con- 
sists of  eutectic  pearlite  with  crystals  of  Fe3C. 

Upon  the  fields  of  the  diagram,  Fig.  22,  are  given  the 
forms  there  taken  by  associated  iron  and  carbon.  Thus 
above  the  lines  AD  and  DE  there  can  be  only  liquid  solu- 
tion. To  the  right  of  DE  (since  the  line  DE  gives  the 


120  MATERIALS  OF  MACHINES 

saturation  limit)  carbon  in  excess  of  saturation  must  exist 
separately  —  usually  as  graphite  —  in  fact  in  this  field 
graphite  separates  by  gravity  and  floats  on  the  surface, 
thus  carrying  the  residual  mass  back  to  the  saturation 
line.  Fe3C  is  also  present  in  this  field  since  it  separates 
first  and  since  change  into  the  more  stable  graphite  occurs 
very  slowly. 

Cooling  lines  from  the  field  ADE  enter  the  field  FBPR 
through  the  eutectic  point  D,  carrying  eutectic  with 
amounts  of  graphite  and  cementite  that  vary  according 
to  the  location  of  the  starting  point  of  the  cooling  curve. 

The  line  PR  has  been  carefully  located  where  graphite, 
G,  and  cementite,  Fe3C,  interchange  stability.  Above  this 
line  cementite  tends  to  rapid  change  into  graphite  (giving 
up  its  combined  iron),  while  below  this  line  graphite  tends 
to  become  cementite  slowly,  taking  up  the  required  iron. 
Below  the  line  MS  iron  cannot  exist  in  the  7  form,  but 
changes  into  the  a  form,  carrying  graphite  (unstable)  and 
cementite  (stable). 


CHAPTER  VIII 
CAST  IRON 

THE  blast-furnace  produces  different  grades  of  pig 
iron  that  are  usually  numbered  from  1  to  6,  although 
the  numbering  scheme  varies  in  different  countries  and 
in  different  localities  in  the  same  country.  When  the 
product  of  the  blast-furnace  has  been  melted  in  a  foundry 
cupola,  or  a  reverberatory  furnace,  cast  into  sand  molds 
and  cooled,  it  is  called  cast  iron. 

Cast  iron  usually  contains  carbon,  silicon,  manganese, 
sulphur  and  phosphorus;  its  physical  properties  differ 
from  those  of  pure  iron  because  of  the  presence  and  inter- 
action of  these  substances. 

Carbon  is  usually  present  in  cast  iron  in  two  forms: 
(a)  as  pure  carbon  in  the  form  of  distributed  crystals  of 
graphite,  and  (6)  as  combined  carbon  in  the  form  of  dis- 
tributed crystals  of  cementite,  Fe3C.  These  forms  are 
usually  called  graphite  and  combined  carbon. 

Cementite  is  an  intensely  hard  and  very  brittle  sub- 
stance, and  the  influence  upon  iron  of  a  very  small  propor- 
tion of  carbon  in  this  form  is  very  great.  This  is  partly 
due  to  the  fact  that  a  small  weight  of  carbon  makes  a 
large  weight  of  Fe3C.  Taking  the  atomic  weight  of  carbon 
as  12  and  of  iron  as  56  the  ratio  of  weight  of  cementite 

(56  X  3)  I  12 

to  the  contained  carbon  equals *  —  =  15;  hence 

Y2i 

iron  that  has  one  per  cent  by  weight  of  combined  carbon 
may  really  have  15  per  cent  by  weight  of  Fe3C.     Since 

121 


122 


MATERIALS  OF   MACHINES 


the  specific  gravity  of  iron  is  7.8  and  of  Fe3C  is  7.07,  it 
follows  that  the  volume  of  Fe3C  in  the  case  just  cited  is 
16.3  per  cent. 

Percentages  by  weight  and  by  volume  of  Fe3C  in  iron 
with  varying  carbon  may  be  tabulated  as  follows: 


Per  cent  weight 
carbon 

Per  cent  weight 
Fe3C 

Per  cent  volume 
Fe3C 

1 

15 

16.3 

2 

30 

32.1 

3 

45 

47.4 

4 

60 

62.3 

4.5 

67.5 

69.6 

These  values  result  from  the  assumption  that  only  iron 
and  carbon  are  present  and  they  would  of  course  be  modi- 
fied in  the  case  of  actual  cast  iron;  but  they  show  how 
great  an  influence  a  small  amount  of  combined  carbon 
may  have  on  cast  iron. 

Since  cementite  is  hard  and  brittle,  its  presence  in  large 
proportion  gives  similar  qualities  to  cast  iron.  In  fact, 
white  cast  iron,  in  which  carbon  is  chiefly  in  the  combined 
state,  is  weak  in  tension,  though  strong  in  compression; 
it  is  so  hard  that  it  can  only  be  machined  by  grinding; 
it  is  also  brittle,  that  is,  it  has  very  low  ductility  and 
resilience;  but  it  has  great  resistance  to  wear  and  hence 
it  is  produced  by  " chilling"  in  the  surfaces  of  rolls  for 
rolling  steel  and  in  the  treads  of  cast-iron  car  wheels. 

Graphite  is  an  allotropic  form  of  carbon  and  when  it 
is  present  in  cast  iron  it  produces  a  gray  fracture  and  inter- 
rupts the  continuity  of  the  iron  structure  and  thus  re- 
duces the  strength  of  the  iron  with  which  it  is  associated. 

Conversion  of  the  carbon  of  the  Fe3C  of  white  cast  iron 
into  the  graphite  of  gray  cast  iron  leaves  the  iron  of  the 
Fe3C  (which  may  be  a  large  proportion  of  the  mass)  in 


CAST  IRON  123 

the  a  form  and  thus  substitutes  a  ductile  material  strong 
in  tension  for  a  brittle  one;  moreover,  both  the  iron  and 
graphite  are  soft  and  hence  the  change  from  white  iron  to 
gray  iron,  except  in  extreme  cases,  is  accompanied  by  in- 
creased tensile  strength  and  ductility  and  softness;  the 
compressive  strength  is  decreased.  Hence  means  for 
control  of  carbon  between  the  states  of  cementite  and 
graphite  are  of  great  importance. 

The  total  carbon  in  cast  iron  is  almost  always  within 
the  limits  2.6  per  cent  and  4.6  per  cent;  therefore,  in 
Fig.  23,  the  lines  QQi  and  HHi  bound  the  cast-iron  field 
on  the  equilibrium  diagram. 

Molten  iron  associated  only  with  carbon,  ready  for 
casting,  would  be  represented  by  some  point  in  the  field 
above  qDr  at  (say)  2400°  F.  If,  during  melting  in  contact 
with  carbon  fuel,  the  iron  had  become  saturated  with  car- 
bon —  as  often  occurs  —  the  cooling  point  would  be  at 
w  and  during  cooling  either  in  the  ladle  or  mold,  Fe3C 
would  separate  and  with  sufficient  time  the  carbon  would 
change  to  graphite  which  would  float  as  long  as  the  mass 
remained  fluid  enough  for  gravity  to  cause  the  separation, 
and  afterward  would  be  irregularly  distributed  throughout 
the  mass.  Thus  the  cooling  point  would  follow  wD  to  D 
where  the  eutectic*  would  be  formed,  and  this  eutectic 
would  be  more  or  less  uniformly  mixed  with  the  graphite 
that  separated  along  wD  through  the  Fe3C  form.  Doubt- 
less also  some  Fe3C  would  remain  unconverted. 

As  very  slow  cooling  goes  on  the  line  PR  is  reached 
where  the  stability  of  G  and  Fe3C  interchange,  and  there 
is  a  tendency  for  the  graphite  formed  to  take  up  iron  and 

*  X  (C)  with  4.5  carbon  in  liquid  solution  changing  into  crystals 
of  X  (C)  with  2.1  per  cent  carbon  in  solid  solution,  intimately  as- 
sociated with  crystals  of  graphite  corresponding  to  the  residue  of 
the  4.5  per  cent  carbon. 


124                     MATERIALS 

OF  MACHINES 

Q 

H 

2800 
2600 
2400 

2200 

2050 
2000 

1800 

^•1660 

£1600 

p. 
§1400 
ft  1330 

1200 
1000 
800 
600 
400 
200 
0 

A 

E 

\ 

\ 

/ 

/ 

\ 

^^-^ 

\ 

^ 

LIQU 

V 

ID  SO 
t 

-UTIO 
s 

I 

/•' 

,iq.Sol. 
stable) 

\ 

\     7 

J 

I.Sol. 

vi 

/ 

'+Fe3C 

(u 

stable) 

\ 

B 

^ 

^•^ 

/ 

T 

F 

I 

/ 

7(C 

)+ 

- 

itable) 

K 
\ 

/ 

/ 

+ 

Fe 

C( 

unstabl 

\ 

r/ 

R 

/ 

7(1 

;)+ 

CCstab 

X. 

+  c 

(unstab 

) 

3 

M 

O 

a- 

^Fe 

iC 

stable 

hG 

(UE 

istable) 

Q! 

Hi 

0.5       1        1.5       2 

2.5        3        3.5        4       4.5 
Percent  of  Carbon 

5        5.5        6 

46 


FIG.  23. 


to  become  FeaC.  This  tendency,  however,  is  less  if  a  large 
amount  of  graphite  is  present  and  especially  if  it  is  present 
in  large  flakes.  At  the  line  MS,  since  this  represents  the 
lowest  temperature  at  which  y  iron  is  stable,  7  (C),  with 
slow  cooling,  tends  to  change  into  a  iron  and  Fe3C;  but 
here  also  the  presence  of  graphite,  especially  in  large 


CAST  IRON  125 

flakes,  tends  to  oppose  the  change  and  may  compel  the 
decomposition  of  7  (C)  into  a  iron  and  temper  graphite.* 
In  this  case  the  resulting  mass  will  have  with  the  a.  iron  a 
large  amount  of  graphite  as  flakes  and  temper  graphite 
and  a  relatively  small  amount  of  cementite.  It  would 
therefore  be  a  gray  iron. 

But  suppose  that  the  cooling  point  instead  of  starting 
from  w  should  start  from  some  point  v  corresponding  to 
3  per  cent  of  carbon  and  2400°  F.  The  point  would  de- 
scend vertically  to  Vi  and  follow  ViD  with  separation  of 
solid  7  (C),  which  would  be  more  or  less  uniformly  dis- 
tributed throughout  the  mass;  on  reaching  D  the  residual 
liquid  would  be  decomposed  into  the  eutectic  (as  before 
intimately  associated  7  (C)  and  G).  The  resulting  mass, 
therefore,  would  consist  of  eutectic  with  excess  of  7  (C), 
and  with  very  slow  cooling,  in  the  absence  of  excess  of 
large  flake  graphite,  on  reaching  PR  the  graphite  wholly 
or  in  part  would  take  up  iron  and  become  Fe3C.  Hence 
between  PR  and  MS  the  mass  consists  of  7  (C)  +  Fe3C  + 
some  graphite.  At  MS  the  7  (C)  is  decomposed  (in  the 
absence  of  large  flake  graphite)  into  a  iron  and  Fe3C. 
This  mass  would  have  large  excess  of  Fe3C  and  little 
graphite  and  hence  would  be  a  white  or  light  gray  iron. 

Chilling.  —  The  changes  just  considered  are  accom- 
plished slowly  and  hence  rapid  cooling  might  render  them 
incomplete,  while  sudden  cooling  might  suppress  them 
entirely.  If  molten  cast  iron  were  poured  into  an  ample 
bath  of  cold  water,  cooling  would  be  sudden  and  the  cooled 
iron  would  have  a  large  proportion  of  its  carbon  in  solu- 
tion, 7  (C),  because  of  lack  of  time  for  the  change,  while 
a  small  proportion  would  be  combined  in  Fe3C.  Between 
this  extreme  of  sudden  cooling  and  the  other  extreme  of 

*  Temper  graphite  is  graphite  very  finely  divided;  in  fact,  it  is  a 
microscopic  dust,  uniformly  distributed  as  in  malleableized  castings. 


126  MATERIALS  OF  MACHINES 

very  slow  cooling,  the  cooling  rate  can  be  varied  with  the 
production  of  intermediate  results. 

In  casting  chilled  car  wheels,  the  portion  of  the  mold 
that  forms  the  tread  is  of  iron,  which  conducts  heat  away 
rapidly,  while  the  portion  that  forms  the  rest  of  the  wheel 
is  of  sand,  which  conducts  heat  away  slowly.  Thus  the 
tread  is  " chilled"  and,  with  proper  grade  of  iron,  cools 
white,  while  the  rest  of  the  wheel  is  gray,  with  an  inter- 
mediate territory  of  mottled  iron.  This  gives  a  wheel 
with  a  very  hard  tread  to  resist  wear,  a  strong  web  and  a 
hub  that  is  not  only  strong  but  soft  for  machining.  See 
Fig.  24. 


FIG.  24. 

The  rate  of  cooling  of  castings  made  in  sand  molds 
varies.  The  rate  of  outflow  of  heat  is  proportional  to 
the  external  surface  of  the  casting,  while  the  heat  to  be 
removed  to  effect  cooling  is  proportional  to  the  volume  of 
the  casting;  hence  the  rate  of  cooling  depends  upon  the 
ratio  of  surface  to  volume  of  the  casting,  and  it  follows 
that  thick  castings  cool  more  slowly  than  thin  castings. 


CAST  IRON  127 

Hence  the  same  iron  cast  into  thick  and  thin  forms  would 
give  castings  of  different  composition,  structure  and 
properties. 

If  it  is  desired  to  accelerate  cooling,  the  sand  of  the  mold 
which  acts  as  heat  insulation  may  be  removed  as  soon  as 
the  iron  is  cool  enough  to  hold  its  form. 

The  form  of  carbon  in  cast  iron  may  be  controlled  by 
regulation  of  the  quantities  of  other  substances  present. 

Manganese  is  held  in  solid  solution  by  iron  in  a  wide 
range  of  proportions;  it  also  may  combine  chemically 
with  carbon  to  form  a  carbide,  Mn3C,  which  goes  into 
solid  solution  with  Fe3C  to  form  what  may  be  called 
iron-manganese  carbide.  The  carbide  Mn3C  is  hard  and 
brittle  and  its  association  by  solution  with  Fe3C,  which 
is  only  a  little  less  hard  and  brittle,  would  presumably 
produce  a  compound  with  similar  qualities,  which  it 
would  give  in  less  degree  to  the  mass  with  which  it  might 
be  associated.  Moreover,  since  the  manganese  must  take 
up  carbon  to  produce  the  carbide  it  would  reduce  the 
carbon  which,  in  its  absence,  would  appear  as  graphite; 
thus  gray  iron  would  become  whiter. 

Authoritative  tests  seem  to  show  that  addition  of  man- 
ganese to  mild  steel  increases  tensile  strength,  and  that 
ductility,  though  little  changed  up  to  0.5  per  cent,  is 
reduced  with  further  additions  of  manganese  until  at 
2  per  cent  the  steel  is  quite  brittle.  Since  there  is  a  very 
small  amount  of  carbon  in  this  mild  steel,  it  is  probable 
that  these  effects  are  due  to  manganese  in  solution  with 
the  iron  itself. 

Whatever  the  theory  may  be  it  is  certain  that  manga- 
nese up  to  1  per  cent  in  cast  iron  increases  strength  and 
probably  reduces  ductility,  and  that  in  larger  amounts  it 
tends  to  change  graphite  into  combined  carbon. 

If  sulphur  is  present  in  cast  iron  its  form  is  FeS,  and 


128  MATERIALS  OF  MACHINES 

it  has  a  very  decided  tendency  to  cause  graphite  to  become 
combined  carbon.  This  tendency  of  sulphur  is  much 
greater  than  that  of  an  equal  amount  of  manganese  to 
produce  the  same  result.  When  manganese  is  added  to  a 
cast  iron  containing  sulphur  it  takes  sulphur  from  the  FeS 
to  form  MnS.  Consider  now  three  substances:  (a)  iron 
sulphide,  (6)  manganese  sulphide,  (c)  iron  manganese  car- 
bide, (a)  has  greatest  power  to  convert  graphite  into 
combined  carbon;  (c)  is  next  in  order  and  (b)  has  least 
power  to  produce  this  result.  With  a  given  amount  of 
FeS  present  the  addition  of  manganese  in  amount  that 
would  just  take  all  sulphur  to  form  MnS,  would  convert 
a  substance,  FeS,  with  great  power  to  make  gray  iron 
white  into  a  substance,  MnS,  with  small  power  for  this 
result;  and  thus  the  graphite  would  increase  at  the  ex- 
pense of  combined  carbon  and  the  iron  would  become 
grayer.  Further  increase  in  manganese  would  now  pro- 
duce an  increasing  amount  of  iron  manganese  carbide 
with  increasing  tendency  to  make  the  gray  iron  white, 
until  finally  this  tendency  would  just  equal  that  of  the 
sulphur  before  any  manganese  was  added  and  further 
increase  would  make  the  iron  whiter  still.  Obviously, 
the  limits  of  this  reversed  action  depend  on  the  amount 
of  sulphur  present.  With  no  sulphur,  the  effect  of  increas- 
ing amount  of  manganese  would  be  increasing  tendency 
to  whiten  the  iron;  whereas  when  sulphur  is  present,  the 
effect  of  increasing  amount  of  manganese  would  be  first 
to  neutralize  the  sulphur  with  increasing  grayness  of  the 
iron,  and  then  to  turn  the  gray  iron  whiter.  This  ex- 
plains why  a  small  addition  of  ferromanganese  to  molten 
sulphur-iron  may  act  as  a  softener.  Sulphur  is  undesirable 
in  cast  iron,  causing  hardness,  weakness  and  brittleness; 
it  should  not  exceed  0.15  per  cent  and  thus  it  should  not 
be  used  as  a  means  for  regulation  of  the  state  of  carbon. 


CAST  IRON  129 

Manganese,  on  the  other  hand,  within  limits  has  a  desir- 
able effect  upon  strength  and  hence  may  be  used  to  reg- 
ulate carbon. 

Silicon  is  held  in  solid  solution  by  a.  iron  up  to  about 
2  per  cent  and  up  to  this  limit  it  seems  to  increase  the 
strength  and  to  reduce  ductility  of  iron;  beyond  this  limit 
Fe2Si  forms  in  increasing  amount,  and  the  presence  of 
this  silicide  reduces  strength,  ductility  and  shock  resist- 
ance of  cast  iron.  Silicon  also  tends  to  reduce  iron  oxide, 
to  remove  gas  that  causes  porosity  and  to  increase  the 
fluidity  of  the  molten  iron,  so  that  stronger,  denser  and 
sharper  castings  are  produced.  Silicon  also  tends  to 
force  carbon  from  the  combined  to  the  graphitic  form, 
or  to  make  iron  grayer.  Possibly  silicon  that  forms  Fe2Si 
takes  the  iron  for  this  purpose  from  Fe3C,  thus  leaving 
the  carbon  to  take  graphitic  form.  Obviously,  silicon 
would  tend  to  neutralize  the  effect  of  manganese  and 
sulphur  to  make  iron  whiter. 

Ferrosilicon,  as  high  silicon  pig  iron  is  called,  is  now  on 
the  market,  and  its  use  in  the  foundry  cupola  charge  gives 
control  of  the  silicon  in  castings  and  thus  within  limits 
gives  control  of  the  state  of  the  carbon  in  the  cast  iron. 

To  determine  the  effect  of  a  varying  proportion  of  silicon 
upon  cast  iron,  Professor  Thomas  Turner  made  a  series 
of  experiments  which  were  reported  under  the  title  "  In- 
fluence of  Silicon  on  the  Properties  of  Cast  Iron  "  in  the 
Journal  of  the  Chemical  Society  in  1885.  He  used  iron 
with  as  nearly  as  possible  2  per  cent  carbon  and  with 
sulphur,  phosphorus  and  manganese  quite  low,  and  by 
introducing  ferrosilicon  he  was  able  to  produce  test 
pieces  with  desired  proportions  of  silicon.  These  were 
tested  and  results  in  tensile  and  compressive  strength 
and  hardness  are  plotted  in  Fig.  25. 

Progress  from  0   toward   the   right,   with   increasing 


130 


MATERIALS  OF  MACHINES 


tensile  strength  and  decreasing  hardness,  probably  cor- 
responds to  the  combined  effect  of  solution  of  silica  in 
iron,  and  the  influence  of  formation  of  Fe2Si  to  increase 
graphite  at  the  expense  of  combined  carbon. 


10 — *fi  SILICON 


As  has  already  been  stated,  white  iron  is  stronger  in 
compression  than  gray  iron,  and  hence  the  compression 
curve  should  fall  from  the  start  whereas  it  rises  up  to 
about  0.8  per  cent.  This  may  be  due  to  the  direct  ac- 
tion of  the  silicon  upon  the  iron  or  to  the  fact  that  small 
amounts  of  silicon  tend  to  reduce  the  porosity  that  is 
common  in  white  cast  iron  and  to  give  sounder  and 
hence  stronger  castings. 


CAST  IRON  131 

It  is  clear  from  these  tests  that  by  varying  the  silicon 
content  it  is  possible  to  control  very  materially  the  physi- 
cal properties  of  cast  iron.  It  would  be  very  helpful  to 
know  what  the  effect  of  increasing  silicon  would  be  with 
the  total  carbon  higher  than  2  per  cent  to  correspond 
more  nearly  to  foundry  practice. 

Phosphorus  probably  goes  into  solid  solution  in  iron 
in  amounts  such  as  are  usually  present  in  steel;  but  as  it 
increases  to  the  values  common  in  cast  iron  a  large  part 
of  it  becomes  Fe3P.  Phosphorus  may  affect  cast  iron  in 
three  ways:  (a)  directly,  by  solution  in  the  iron;  (6)  indi- 
rectly, by  its  power  to  cause  graphite  to  become  combined 
carbon;  (c)  indirectly,  because  its  presence  causes  cooling 
iron  to  pass  through  a  pasty  state  thus  delaying  solidifi- 
cation. This  delay  makes  it  possible  for  tendencies  that 
are  active  in  the  cooling  mass  to  produce  more  complete 
results.  Thus  with  high  silicon  the  presence  of  phos- 
phorus —  by  delaying  solidification  —  might  enable  the 
silicon  to  produce  a  larger  proportion  of  graphite,  thus 
giving  a  softer  iron  stronger  in  tension  in  spite  of  the  op- 
posite effect  of  the  phosphorus  itself.  Obviously  the  re- 
sultant effect  would  depend  on  the  relative  amounts  of 
the  silicon  and  phosphorus  present  and  with  low  silicon 
and  high  phosphorus  the  effect  would  undoubtedly  be  to 
whiten  the  iron,  (a)  and  (6)  produce  harmful  increase  in 
hardness  and  brittleness;  (c)  in  the  presence  of  silicon 
may  produce  a  desirable  increase  in  strength  and  soft- 
ness. Again,  whatever  theory  may  be  right,  it  is  a  fact 
that  although  phosphorus  is  a  very  detrimental  constitu- 
ent in  steel,  it  is  not  harmful  in  cast  iron,  often  being 
present  up  to  1.3  per  cent.  Phosphorus  is  useful  in  cast 
iron  because  it  increases  the  fluidity  of  the  molten  mass 
so  that  sharp  castings  can  be  made  with  lower  casting 
temperature. 


132  MATERIALS  OF  MACHINES 

Semi-steel.  —  There  is  another  method  of  carbon 
control  that  gives  excellent  castings.  In  case  of  cast  iron 
with  low  total  carbon,  if  all  or  nearly  all  of  the  carbon 
could  be  caused  to  appear  as  very  finely  divided  graphite 
-  like  temper  graphite  in  malleableized  castings  —  there 
would  be  very  low  cementite  and  relatively  small  inter- 
ference with  the  continuity  of  the  iron  by  the  graphite, 
and  hence  the  iron  would  be  strong  and  soft. 

In  many  foundries  it  is  now  customary  to  introduce 
with  the  regular  cupola  charge  about  25  per  cent  of  steel 
scrap,  which  mixes  with  the  cast  iron  and  melts.  Ne- 
glecting the  carbon  of  the  steel,  and  assuming  the  total 
carbon  of  the  rest  of  the  charge  to  be  4  per  cent  it  follows 
that  the  total  carbon  would  be  reduced  by  dilution  to 
3  per  cent.  Or  in  Fig.  23  the  starting  point  of  cooling 
would  be  moved  from  s  to  v.  Cooling  slowly  from  v  with 
only  carbon  and  iron  present  would  give  a  white  iron. 
Addition  of  sulphur,  phosphorus  and  manganese  in  cus- 
tomary amounts  would  leave  the  iron  still  white;  but 
addition  of  silicon  in  sufficient  quantity  could  cause  most 
of  the  carbon  to  appear  as  finely  divided  graphite  (since 
the  total  carbon js  low),  giving  a  soft  strong  iron.  It  is 
interesting  to  note  that  there  is  a  possible  ideal  relation 
of  quantity  of  these  substances;  viz.,  with  a  certain  neces- 
sary amount  of  sulphur  there  should  be  enough  manga- 
nese to  convert  all  of  the  FeS  to  MnS.  The  total  carbon 
should  be  kept  low  enough  by  dilution  so  that  when  the 
greatest  possible  proportion  is  converted  into  graphite, 
it  shall  appear  as  very  fine  grains  instead  of  as  flakes. 
The  amount  of  silicon  should  be  sufficient  to  provide  for 
the  fluxing  that  removes  iron  oxide  completely  and  pre- 
vents porosity,  and  also  by  solution  or  chemical  combi- 
nation to  cause  the  maximum  change  of  combined  carbon 
into  finely  divided  graphite.  There  should  be  just  enough 


CAST   IRON  133 

phosphorus  to  delay  solidification  for  the  silicon  to  produce 
its  best  result  without  too  great  phosphorus  effect  on 
carbon  and  iron.  It  would  probably  be  impossible  to 
produce  this  ideal  result,  but  it  is  true  that  the  so-called 
semi-steel  often  has  tensile  strength  above  30,000  pounds 
per  square  inch,  together  with  close  grain  and  good  wear- 
ing resistance  and  softness  for  easy  machining. 

Aluminum  introduced  into  molten  cast  iron  produces 
two  results.  A  part  combines  with  oxygen  to  form  alu- 
mina, thus  reducing  undesirable  oxide  of  iron  and  absorb- 
ing gas  that  would  cause  porosity.  The  alumina  thus 
formed  combines  with  other  waste  and  forms  either  a 
fusible  slag  or  an  infusible  crust  which  is  removed.  An- 
other part  of  the  aluminum  —  if  there  is  an  excess  —  may 
combine  with  the  iron,  and  when  it  does,  as  shown  by 
Mr.  J,  W.  Keep,*  its  influence  upon  the  distribution  of 
carbon  is  similar  to  that  of  silicon.  But  although  ferro- 
aluminum  is  on  the  market,  it  is  not  used  as  a  softener  of 
cast  iron  because  it  is  more  expensive  than  ferrosilicon, 
which  produces  the  same  result;  moreover,  when  alu- 
minum is  used,  a  skin  that  forms  on  the  surface  of  the 
molten  iron  tends  to  cause  "cold  shuts  "  and  defective 
casting  surfaces. 

Malleable  cast  iron.  —  Theory  of  mallifying  process 
for  production  of  malleable  cast  iron. 

An  average  composition  of  castings  for  mallifying  is  as 

follows : 

Per  cent 

Total  carbon 2.75 

Silicon 0.8 

Manganese 0.4 

Phosphorus 0.17 

Sulphur,  under 0 . 05 

*  See  Transactions  Am.  Inst.  Mining  Engineers,  Vol.  XVIII, 
p.  102. 


134  MATERIALS  OF  MACHINES 

These  castings  with  low  carbon,  low  silicon  and  relatively 
high  manganese  and  phosphorus,  and  with  the  relatively 
quick  cooling  which  corresponds  to  malleable  iron  foundry 
practice,  hold  all  carbon  in  the  combined  state;  that  is, 
the  casting  fractures  white.  In  the  mallifying  process  the 
castings,  packed  in  iron  oxide  or  other  material,  are  raised 
to  a  temperature  of  from  1500°  to  1600°  F.  This  brings 
them  into  the  lower  portion  of  the  field  (Fig.  23)  bounded 
by  the  lines  QQi,  HHi  and  BF,  PR,  where  graphite  is 
stable  and  where  FesC  is  unstable.  In  passing  the  line 
OS  upward  the  a  iron  changes  into  7  iron  and  takes  carbon 
into  solution  to  form  7  (C)  ;  but  the  amount  of  carbon 
available  is  small,  since  Fe3C  is  stable  in  this  field,  and 
hence  probably  7  (C)  holds  less  than  1  per  cent  of  carbon 
in  solution.  The  castings  are  held  in  the  field  above  PR 
for  about  60  hours,  and  this  time  is  sufficient  for  the 
stability  tendencies  to  reach  equilibrium,  and  most  of  the 
Fe3C  gives  up  its  carbon  to  form  temper  graphite  which 
appears  as  uniformly  distributed  microscopic  dust.*  The 
iron  thus  isolated  in  this  field  by  decomposition  of  FesC 
is  in  the  7  form,  and  it  takes  silicon  and  carbon  into  solu- 
tion, forming  7  (Si,  C).  If  no  silicon  were  present  7  (C) 
would  be  formed,  but  when  silicon  is  present  it  crowds  out 
and  replaces  a  part  of  the  carbon  of  7  (C)  giving  7  (Si,  C). 
Hence  the  greater  the  amount  of  silicon  within  limits  the 
greater  the  amount  of  carbon  that  appears  as  temper 
graphite.  The  result,  therefore,  of  holding  the  castings 
in  this  field  is  production  of  7  (Si,  C)  with  low  carbon, 
associated  with  temper  graphite  and,  undoubtedly,  with 
a  small  amount  of  unchanged 


*  Probably  the  carbon  takes  this  form  here  because  the  mass  is 
resistant  to  the  migration  of  the  carbon.  When  the  eutectic  is  formed 
at  D,  the  solidifying  mass,  being  less  resistant,  permits  the  separating 
graphite  to  migrate  through  the  mass  and  to  unite  into  graphite 
flakes. 


CAST  IRON  135 

When  the  slow  cooling  takes  place,  on  passing  OS  the 
7  iron  of  the  7  (Si,  C)  changes  back  to  a  iron,  releasing 
the  carbon  but  holding  the  silicon  in  solid  solution,  a  (Si).* 
The  released  carbon  in  the  presence  of  the  a  (Si)  and  of 
the  temper  graphite  already  formed  is  forced,  in  opposition 
to  the  equilibrium  tendency  of  this  field,  to  become  temper 
graphite,  and  thus  the  mallifying  process  is  complete  and 
the  cooled  castings  are  strong  and  ductile  with  a  fracture 
that  is  black  with  temper  graphite  with  a  thin  white  skin. 
The  reasoning  given  applies  only  to  the  interior  portion 
of  the  castings,  since  the  white  skin  that  is  formed  on  the 
castings  during  mallifying  has  its  total  carbon  notably 
reduced  by  oxidation  either  by  the  oxygen  of  the  air 
trapped  in  the  packing,  or  by  the  oxygen  of  the  oxide 
packing  itself,  thus  becoming  a  skin  of  something  like 
mild  steel.  The  ductility  of  this  skin  is  of  great  impor- 
tance in  the  malleable  castings,  and  its  removal  sensibly 
diminishes  their  value  as  stress  members  of  machines; 
hence  the  stress-parts  of  malleable  castings  are  seldom 
machined. 

The  "  white  heart  "  malleable  castings  of  England  and 
the  continent  of  Europe  consist  of  light  or  very  thin  cast- 
ings that  are  almost  completely  decarbonized,  like  the  skin 
of  the  American  "black  heart  "  castings,  by  higher  malli- 
fying temperature  and  extension  of  the  time  of  mallify- 
ing. 

Shrinkage.  —  When  molten  cast  iron  is  poured  into 
a  mold  it  takes  the  form  of  the  mold  and  cools  gradually 
to  the  temperature  of  the  surrounding  air.  Shrinkage 
which  accompanies  cooling  may  be  divided  into  fluid 
shrinkage  and  solid  shrinkage. 

As  the  molten  iron  in  the  mold  after  casting  begins  to 
cool,  it  shrinks  in  volume.  This  shrinkage  may  be  "fed 

*  Some  silicon  is  also  in  solid  solution  with  the  remaining  Fe3C. 


136  MATERIALS  OF   MACHINES 

from  a  riser/'  *  until  the  connection  is  frozen  up.  The 
walls  of  the  casting  solidify,  but  at  first  are  weak  and 
yield  to  the  shrinkage  of  the  still  fluid  iron  within  the 
casting;  if  the  volume  of  the  casting  is  large,  depressions 
in  the  walls  result.  Later  the  walls  become  rigid  enough 
to  resist  shrinkage  of  the  remaining  fluid  within,  and 
since  the  volume  cannot  be  reduced  further,  portions  of 
the  mass  pull  apart  and  the  casting  becomes  spongy.  A 
spongy  cross  section  is  necessarily  weaker  than  one  of 
solid  iron,  and  is  therefore  undesirable  in  a  machine 
stress  member.  Evidently  the  tendency  to  form  spongy 
iron  because  of  unsupplied  fluid  shrinkage  increases  with 
the  volume  of  the  casting. 

Experience  points  to  the  conclusion  that  castings  of 
small  cross  section  shrink  more  than  those  of  large  cross 
section.  To  test  this  conclusion,  Mr.  Thomas  D.  West 
made  an  experiment,  which  he  describes  in  his  book 
"  American  Foundry  Practice."  He  cast  two  bars  14 
feet  long,  from  the  same  iron,  and  as  far  as  possible  made 
the  conditions  of  casting  the  same  for  both.  The  cross 
sections  were  rectangular,  one  being  4  inches  by  9  inches 
and  the  other  J  inch  by  2  inches.  The  total  shrinkage 
for  the  larger  bar  was  f  inch  and  for  the  smaller  one  was 
If  inches.  This  may  possibly  be  explained  as  follows, 
as  Mr.  West  suggests:  A  casting  cools  from  the  surface, 
and  therefore  during  the  cooling  the  surface  will  be  the 
coolest  part,  and  the  heat  will  increase  toward  the  center. 
The  external  portions  are  held  from  their  normal  shrinkage 

*  A  "riser"  is  formed  by  making  a  vertical  cylindrical  opening 
in  the  sand  which  connects  with  the  main  portion  of  the  mold. 
The  molten  iron  during  pouring  rises  in  this  cylindrical  opening  to 
form  the  riser.  If  the  riser  is  large  in  proportion  to  the  casting  it 
remains  fluid  for  some  time  and  acts  as  a  reservoir  to  supply  fluid 
shrinkage.  Molten  iron  from  a  ladle  may  be  fed  in  to  maintain 
the  level  in  the  riser. 


CAST  IRON  137 

by  the  resistance  of  the  hotter  internal  portions,  which 
are  not  yet  ready  to  shrink  as  much.  This  goes  on  until 
the  surface  has  reached  the  temperature  of  the  surround- 
ing air  and  stops  shrinking;  the  hotter  portions  nearer 
the  center  now  try  to  shrink  as  they  in  turn  cool  down, 
but  are  prevented  by  the  external  part  which  has  stopped 
shrinking,  or  it  may  be  that  since  the  thicker  portion 
cools  more  slowly  than  the  thinner  portion,  it  will  be  grayer 
and  the  formation  of  graphite  reduces  the  natural  shrink- 
age of  the  iron.  Whatever  theory  is  correct,  the  fact 
remains  that  castings  of  small  section  shrink  more  than 
castings  of  large  section.  It  follows  that  castings  having 
thick  and  thin  parts  attached  to  each  other  will  shrink 
unequally,  and  be  in  a  state  of  internal  stress,  which 
renders  them  less  able  to  withstand  the  action  of  external 
forces. 

Suppose  it  is  required  to  put  a  strengthening  rib  B  on  A , 
Fig.  26  (a),  and  that  it  is  made  of  the  form  shown,  i.e., 
thin  relatively  to  A,  and  having  parallel  sides.  B  would 
shrink  more  than  A,  and  shrinkage  stresses  (tension  in 
B  and  compression  in  A)  would  result,  which  would  be 
concentrated  along  the  juncture  of  A  and  B}  and  yield- 
ing would  occur  under  a  less  external  force.  If  the  form 
shown  in  (b)  were  used,  where  the  rib  tapers  from  the 
thickness  of  B  to  the  thickness  of  A,  the  shrinkage  stresses 
would  be  distributed,  and  the  casting  would  be  stronger. 

The  lessons  to  be  learned  from  these  facts  are  as  follows : 
(1)  All  parts  of  all  cross  sections  of  castings  for  machine 
members  should  be  as  nearly  of  the  same  thickness  as 
possible,  to  avoid  concentrated  shrinkage  stresses,  with 
their  accompanying  weakness.  (2)  If  it  is  necessary  to 
have  thick  and  thin  parts  in  the  same  casting,  change  of 
form  from  one  to  the  other  should  be  as  gradual  as  possible. 
(3)  Castings  should  be  made  as  thin  as  is  consistent  with 


138 


MATERIALS  OF   MACHINES 


strength,  stiffness  and  resistance  to  vibration,  to  avoid 
the  shrinkage  stresses,  and  spongy  metal  due  to  the  shrink- 
age of  large  masses.  (4)  Since  some  shrinkage  stresses 
always  must  exist  in  cast  machine  members,  they  should 
be  taken  into  Account  in  designing. 

Special  care  should  be  taken  in  the  design  of  wheels, 
because  they  are  peculiarly  liable  to  excessive  shrinkage 
stress  on  account  of  their  form.  In  a  pulley,  the  thin 
rim  tends  to  shrink  more  than  the  heavier  arms,  and  the 
rim  is  thereby  put  in  tension,  and  the  arms  in  compression. 
It  is  not  uncommon  to  see  a  rim  ruptured  in  this  way. 
If  the  same  pulley  has  a  relatively  heavy  hub,  the  latter 
will  remain  fluid  until  the  arms  and  rim  have  solidified; 
the  tension  on  the  rim  will  then  force  the  arms  into  the 
yet  fluid  hub,  which  in  turn  shrinking,  will  put  the  arms 
in  tension.  The  arms  of  fly-wheels  tend  to  shrink  away 
from  the  heavier  rim,  and  are  therefore  in  tension. 

White  iron  shrinks  more  than  gray  iron,  and  the  reason 
is  obvious.  When  graphite  is  formed  from  the  7  (C)  or 
Fe3C  a  substance  of  low  density  replaces  a  substance  of 
high  density  and  the  volume  of  the  mass  is  thereby  in- 
creased. 

TABLE   OF   DENSITIES  OF   DIFFERENT   GRADES  OF  CAST  IRON  * 


Grade  of  iron 

Specific  gravity 

Pure  iron 

7   86 

White  cast  iron  
Mottled  cast  iron  
Light  gray  cast  iron  

7.60 
7.35 
7.20 

Dark  gray  cast  iron  

6.80 

*  Abridged  from  "The  Metallurgy  of  Iron  and  Steel"  by  Bradley  Stoughton 
(McGraw-Hill  Book  Co.). 

The  white  castings  for  mallifying  shrink  more  than  gray 
castings;  but  during  mallifying,  as  a  result  of  the  appear- 


CAST  IRON  139 

ance  of  low-density  graphite,  the  castings  expand.  The 
resultant  shrinkage  due  to  the  entire  process  for  production 
of  malleable  cast  iron  is  about  the  same  as  the  shrinkage 
of  gray  castings. 

Effect  of  internal  stress  upon  the  strength  of  cast- 
ings. —  Suppose  that  a  casting  is  made  of  the  cross- 
sectional  form  shown  in  Fig.  26  (a).  The  part  B  tends  to 
shrink  more  than  A,  and,  therefore,  B  is  put  in  tension 
and  A  is  put  in  compression.  Where  there  is  compressive 
stress,  and  tensile  force  is  applied,  the  first  effect  is  the 


FIG.  26. 

reduction  of  the  compressive  stress  to  zero.  No  tensile 
stress  can  be  induced  until  the  compressive  stress  is  entirely 
neutralized.  If  a  tensile  force  is  applied  to  the  casting 
(a),  Fig.  26,  it  follows  that  no  tensile  stress  will  result  in 
the  part  A,  and,  therefore,  that  all  the  stress  will  be  con- 
centrated on  the  part  B.  To  illustrate  this,  suppose  that 
a  tensile  force  is  applied  to  a  rope,  and  that  half  of  the 
strands  are  tight,  and  the  other  half  are  slack.  Stress 
will  result  in  the  strands  which  are  tight  until  they  are 
strained  so  much  that  the  others  are  brought  into  play, 
and  then  the  tension  is  sustained  by  the  whole  cross 
section,  provided  the  strands  originally  tight  are  not 
broken.  In  the  casting,  the  part  B  sustains  the  stress 
until  the  compression  in  A  is  neutralized,  and  its  tensile 
resistance  is  brought  into  play.  Because  of  this  the  unit 
stress  (stress  per  unit  of  cross-sectional  area  sustaining 
the  stress)  is  very  great  in  the  early  part  of  the  test,  and 
the  deformation,  having  a  proportionate  value,  is  also 


140  MATERIALS  OF   MACHINES 

much  greater  than  it  would  be  if  the  whole  area  of  cross 
section  sustained  the  stress.  The  stress-deformation 
diagram,  therefore,  takes  the  form  shown  in  Fig.  27;  the 
initial  part  of  the  curve  representing  the  concentration 
of  stress  on  some  fraction  of  the  cross-sectional  area. 
If  the  stress  had  been  gradually  re- 
lieved at  A,  the  curve  would  have 
returned  over  AB,  and  OB  would  be 
the  permanent  deformation  or  "set." 
If  the  internal  stress  in  B}  Fig.  26, 
had  been  sufficiently  great,  it  might 
have  been  ruptured  before  the  tensile 
resistance  of  A  could  be  brought  into 
action.  In  any  case,  the  piece  could 


O  ^B 

FIG  27  not  sustain  as  great  external  force  as 

if  there  had  been  no  internal  stress, 
because  there  would  be  no  time  during  the  application  of 
force  when  the  whole  area  of  cross  section  would  offer  resist- 
ance without  some  part  having  been  previously  weakened. 

A  varied  quality  of  product  is  required  from  a  foundry. 
The  most  important  requirement  for  some  castings  that 
are  not  subjected  to  any  considerable  stress  is  that  they 
shall  "run  sharp  ";  that  is,  that  they  shall  take  and  retain 
the  form  of  the  mold  accurately.  A  very  gray  silicon 
iron  with  its  low  shrinkage,  and  with  phosphorus  enough 
to  give  fluidity  at  casting  temperature  would  serve. 

Other  castings  require  to  be  as  strong  as  possible  in 
tension  because,  though  cast  iron  is  seldom  used  in  direct 
tension,  machine  members  are  often  subjected  to  bending 
forces  which  cause  both  tensile  and  compressive  stress. 
Still  other  castings  require  great  compressive  strength  or 
great  compressive  shock-resisting  capacity;  as,  for  in- 
stance, anvil  blocks  for  power  hammers. 

Professor  Thomas  Turner  in  a  paper  in  the  Trans- 


CAST  IRON 


141 


actions  of  the  Iron  and  Steel  Institute  1885,  recorded  a 
study  of  all  available  data  to  determine  the  best  composi- 
tion of  cast  iron  for  given  requirements.  This  study 
seems  to  indicate  that  for  greatest  softness  combined 
carbon  should  equal  0.15  per  cent,  graphite,  3.1  per  cent. 
To  obtain  this  distribution  with  other  substances  of 
average  values  requires  2.5  per  cent  silicon.  For  great- 
est general  and  tensile  strength  combined  carbon  should 
equal  about  0.5  per  cent,  graphite  from  2.8  to  3  per  cent, 
which  corresponds  to  silicon  from  1.4  to  1.8  per  cent.  For 
greatest  crushing  strength  combined  carbon  should  be 
over  1  per  cent,  graphite  under  2.6  per  cent,  which  cor- 
responds to  silicon  about  0.8  per  cent. 

In  foundry  practice  it  is  desirable  to  use  a  large  amount 
of  "scrap";  partly  because  "sprues,"  "gates,"  "risers," 
etc.,  are  a  necessary  product  of  every  heat,  and  partly 
because  a  good  deal  of  scrap  is  offered  for  sale  at  a  low 
price.  The  effect  of  remelting  iron  is  to  harden  it,  and 
therefore,  scrap  is  always  of  harder  grade  than  the  "pig" 
from  which  it  was  originally  cast. 

The  hardening  effect  of  remelting  is  very  clearly  shown 
by  some  experiments  made  at  the  Gleiwitz  foundry  in 
Silesia,  and  quoted  by  M.  Ferd.  Gautier  in  a  paper  read 
before  the  Iron  and  Steel  Institute  (see  Journal  of  1886). 
The  results  are  given  in  the  following  table: 


Substances  with  the  iron 

Original  pig 
iron 

After  fourth 
casting 

After  sixth 
casting 

Graphitic  carbon  

2  73 

2  54 

2  08 

Combined  carbon  

0  66 

0.80 

1  28 

Total  carbon  

3.39 

3.34 

3  36 

Silicon    

2.42 

1.88 

1.16 

Manganese  

1.09 

0.44 

0.36 

Sulphur  

0.04 

0.10 

0.20 

Phosphorus 

0  31 

0  30 

0  30 

142  MATERIALS  OF   MACHINES 

Thus,  the  six  successive  meltings  resulted  in  a  decrease 
in  the  amount  of  silicon  and  manganese,  and  an  increase 
in  the  amount  of  sulphur.  (This  latter  probably  was  ab- 
sorbed from  the  fuel.)  Graphitic  carbon  is  decreased  and 
combined  carbon  is  increased;  therefore,  the  combined 
effect  of  decrease  of  silicon  and  increase  of  sulphur  was 
greater  than  the  effect  of  the  decrease  in  manganese.  The 
change  necessary  to  convert  this  again  into  soft  gray  iron 
is  the  addition  of  silicon,  provided  the  amount  of  sulphur 
is  not  too  great.  The  reasons  for  the  hardening  effect 
of  remelting  are :  (a)  the  reduction  of  the  silicon,  resulting 
in  the  redistribution  of  carbon;  (b)  the  increase  of  sul- 
phur. Of  the  substances  which  are  found  in  combination 
with  iron,  silicon  is  first  oxidized,  manganese  being  next 
in  order.  Therefore,  when  iron  is  melted  in  the  presence 
of  an  air  blast,  some  of  the  silicon  is  always  oxidized, 
and  usually  some  of  the  manganese.  Iron  is  melted  in 
the  presence  of  anthracite  coal  or  coke,  and  hence,  there  is 
the  possibility  of  absorption  of  sulphur.  If  the  total 
carbon  is  sufficiently  high,  the  softening  of  iron  can  be 
accomplished  very  satisfactorily  by  the  addition  of  a 
proper  amount  of  ferrosilicon,  which  usually  contains 
about  10  per  cent  of  silicon.  But  if  total  carbon  is  low, 
pig  iron  high  in  silicon  and  carbon  would  serve  better, 
because  it  would  carry  a  large  amount  of  carbon  per  unit 
of  silicon. 

"  Burnt  scrap  "  is  cast  iron  which  has  been  exposed 
during  use  to  the  action  of  oxygen  at  high  temperatures; 
as,  for  instance,  old  grate-bars,  salt-kettles,  etc.  A  por- 
tion of  the  iron  becomes  iron  oxide.  When  such  iron  is 
melted,  the  iron  oxide  gives  up  its  oxygen  to  the  silicon, 
manganese  or  carbon  present,  in  obedience  to  the  law  of 
affinities;  and  the  results  are  silica  and  oxide  of  manganese, 
solids  which  appear  as  slag,  and  the  gas,  carbon  monoxide 


CAST   IRON  143 

or  carbon  dioxide.  The  reduction  of  the  total  carbon 
will  result  in  harder  iron,  and  the  reduction  of  the  silicon 
will  result  in  the  appearance  of  all  the  carbon  present  as 
combined  carbon.  This  result  is  so  very  decided  that  a 
whole  heat  may  "run  hard  "  because  of  the  introduction 
of  a  comparatively  small  amount  of  "burnt  scrap."  If 
the  effect  of  burnt  scrap  is  due  simply  to  the  fact  that 
the  silicon  has  been  removed  by  the  oxygen  of  the  iron 
oxide,  then  if  it  were  melted  together  with  a  sufficient 
amount  of  ferrosilicon,  the  result  would  be  gray,  soft  iron. 
But  there  might  be  iron  oxide  enough  present  to  reduce 
the  total  carbon  too  much;  then  the  silicon  could  not 
produce  gray  iron,  because  it  would  not  have  enough 
carbon  to  work  with;  in  this  case,  carbon  as  well  as  sili- 
con would  have  to  be  added,  and  pig  iron  high  in  carbon 
and  silicon  would  serve  better  than  ferrosilicon.  The 
iron  oxide,  which  is  seen  as  rust  on  the  surface  of  scrap,  is 
effective  in  the  reduction  of  silicon,  etc.,  upon  melting; 
its  effect  is  of  little  importance,  however,  as  it  is  small  in 
amount  relatively.  It  must  not  be  concluded  from  this 
that  silicon  will  make  good  iron  out  of  all  kinds  of  scrap. 
Some  scrap  is  hopeless  because  of  the  presence  of  sulphur 
or  phosphorus.  It  must  be  remembered  that  the  addition 
of  silicon  to  very  gray  iron  can  produce  no  good  results, 
but  rather  the  reverse,  because  the  carbon  is  already 
graphitic,  and  the  only  effect  of  the  addition  of  silicon  is  its 
undesirable  effect  on  the  iron  itself. 


CHAPTER  IX 


STEEL 

IT  has  been  shown  that  steel  is  essentially  a  combination 
of  iron  and  carbon,  which  also  contains  small  amounts 
of  silicon,  manganese,  sulphur  and  phosphorus.  Steel 
also  may  contain  other  substances  such  as  copper,  nickel, 
chromium,  tungsten  and  vanadium,  either  from  the  smelt- 
ing process  or  introduced  because  of  desirable  effect  on 
physical  properties. 

The  chemical  difference  between  cast  iron  and  steel 
is  in  the  amount  present  of  substances  other  than  iron. 
The  following  table  gives  a  chemical  comparison  of  cast 
iron,  the  steel  used  in  structures  and  machines,  and  tool 
steel. 


Substance 

Cast  iron, 
per  cent 

Machinery  and 
structural  steel, 
per  cent 

Tool  steel, 
per  cent 

Carbon  
Silicon 

2.5    to  4.  5 
0  15  to  2  5 

0.10  to  0.6 
0       to  0  04 

0.6      to  1.6 
0  04    to  0  25 

Manganese 

0       to  1  5 

03    to  1  0 

0  23    to  0  5 

Sulphur.  . 

0       to  0  5 

0       to  0  06 

0  002  to  0  012 

Phosphorus  

0       to  1.3 

0.03  to  0.08 

about  0.02 

The  change  from  cast  iron  to  machine  steel  is  effected 
by  removing  by  oxidation  all  substances  other  than  iron, 
as  completely  as  possible  commercially,  and  then  reducing 
iron  oxide,  removing  occluded  gas  and  introducing  the 
required  amount  of  carbon.  Tool  steel  is  made  indirectly 
from  cast  iron  by  more  complete  purification  and  intro- 

144 


STEEL  145 

duction  of  a  larger  proportion  of  carbon.  Some  of  the 
changes  in  physical  properties  that  accompany  these 
chemical  changes  may  be  shown  by  reference  to  Fig.  28. 
AD  is  the  stress  deformation  diagram  of  cast  iron;  AB2D2 
of  machine  steel;  ABiDi  of  high-carbon  or  tool  steel. 
Change  from  cast  iron  to  machine  steel  has  increased 

ultimate  strength  in  the  ratio  2;    it  has  increased 


A  7^ 
ductility  in  the  ratio  -rw',    it  has   increased   ultimate 


,.     area  .,    , 

shock    resistance    in    the    ratio  -  ~ADF  —  ' 

changed  a  material  that  may  be  easily  formed  by  casting 
into  one  that  is  much  more  difficult  to  cast  and  that  is 
usually  formed  by  forging.  More  complete  purification 
and  increase  in  carbon  would  change  machine  steel  into 
tool  steel,  and  strength  would  be  increased  in  the  ratio 

1          ductility  would  be  reduced  in  the  ratio    - 


ultimate  shock  resistance  would  be  changed  in  the  ratio 

A  7?  T)  W 

1    *   *•     This  last  change  in  shock  resistance  might  be 

A  £)2^M^2 

either  an  increase  or  a  decrease,  depending  on  the  amount 
of  carbon  change. 

The  field  of  steel  on  the  equilibrium  diagram,  Fig.  29, 
may  be  assumed  as  limited  on  the  right  by  the  line  JJi 
at  1.7  per  cent  carbon,  although  in  special  cases  the  carbon 
content  is  higher.  Lines  of  slow  cooling  of  steel  may 
now  be  followed  on  this  diagram  from  a,  Oi  and  b  as  on 
page  113. 

First.  Cooling  from  a.  —  Solidification  begins  at  0%  and 
is  complete  at  a3.  The  solid  y  (C)  then  falls  in  temperature 
until  a-i  is  reached,  where  separation  of  /3  iron  begins  and 
continues,  causing  the  point  of  cooling  to  follow  ai^V  to  N. 


146 


MATERIALS  OF  MACHINES 

i       8        S       3       8 


V 


STEEL  147 

At  N  /3  iron  changes  into  a  iron  and  with  further  cooling 
more  of  the  7  iron  of  the  remaining  7  (C)  changes  into  a 
iron,  causing  the  point  of  cooling  to  follow  NO  to  0.  On 
reaching  0  the  mass  consists  of  a  +  7  (C)  +  (probably) 
some  Fe3C.  At  0  the  7  (C)  is  changed  into  the  eutectic 
—  very  intimately  associated  crystals  of  a  iron  and  Fe3C, 
the  mixture  containing  0.9  per  cent  carbon  —  which  is 
associated  less  regularly  and  less  intimately  with  the  a 
iron  that  has  formed  from  N  to  0.  Any  steel  with  less 
than  0.9  per  cent  carbon  will  have  this  qualitative  com- 
position, but  the  greater  the  proportion  of  carbon  the 
greater  the  proportion  of  eutectic  in  the  cooled  steel. 

Second.  Cooling  from  Oi.  —  The  cooling  point  moves 
vertically  from  0\  to  0,  where  the  entire  solid  mass  — 
7  (C)  with  0.9  per  cent  —  changes  directly  into  eutectic, 
(a:  +  Fe3C)o.9  c-     This  cooled  steel  is  a  homogeneous  mass 
of  intimately  associated  small  crystals  of  a.  iron  and  Fe3C. 

Third.  Cooling  from  b.  —  The  cooling  point  moves 
through  62  and  63  and  the  mass  becomes  solid  7  (C) .  At  61 
Fe3C  separates  and  reduction  of  carbon  in  7  (C)  causes  the 
cooling  point  to  follow  b{0  with  continuance  of  the  cement- 
ite  separation.  On  reaching  0,  the  mass  consists  of 
7  (C)o.9c  and  Fe3C;  the  7  (C)o.0c  changes  into  eutectic  and 
the  cooled  mass  is  made  up  of  eutectic  plus  Fe3C.  Slowly 
cooled  steel  with  less  than  0.9  per  cent  carbon  consists  of 
a  plus  eutectic  in  varying  proportion.  Slowly  cooled  steel 
with  0.9  per  cent  carbon  consists  wholly  of  eutectic. 
Slowly  cooled  steel  with  more  than  0.9  per  cent  carbon 
consists  of  eutectic  plus  Fe3C  in  varying  proportions. 

Reference  now  to  Fig.  28  shows  —  since  low-carbon 
steel  is  represented  by  the  diagram  AB^D^Ez  and  high- 
carbon  steel  is  represented  by  the  diagram  ABiDiEi  — 
that  the  presence  of  ferrite,  a  iron,  in  large  proportion 
corresponds  to  high  ductility  and  medium  tensile  strength; 


148 


MATERIALS  OF  MACHINES 


and  that  the  presence  of  cementite,  Fe3C,  corresponds  to 
high  strength  and  medium  ductility.  Also,  the  presence 
of  ferrite  corresponds  to  greater  softness,  while  the  pres- 
ence of  cementite  corresponds  to  greater  hardness  of  the 
mass. 

Oi 


2DUU 

2600 
2400 
2200 

^ 

* 

L 

laid 
olution 

kj/cc 

J 

\lq 
S* 

'dand!^ 
d  Soluti 
V/Y(C) 

0, 
n 

"•+»^ 

^^ 

\ 

& 

^"^> 

^^ 

/ 
/ 
/ 

2000 

Sol 

d  7( 

) 

\ 

B 

***>.  v 

""--.4 

/ 

/ 
/ 
t 

^z 

1660 
1600 
/3+< 

1420 
1400 
1330 

T>nfi 

K 
\ 

I 

/ 

\ 
1C 

7" 

Ol 

^ 

-r/K 

i 

s_      7i 

A 

- 

iT^V^ 

Jz 

R 

K 

«—  a-h 

E—  *• 

Fe3C+l 

-* 

Pearlit 

i  =  Eut 

!ctfe  =  [ 

XCG+I 

e3C)wl 

th  0.9!« 

1UUU 
cnn 

/\ 

5 

Austenite=7  (C) 
CemeBtite^fe8C 
Ferrite     —a  iron 

carjbon 

400 

200 

Ji 

0.5     0.91        1.5 


2          2.5          3         3.5 
FIG.  29. 


4.5          5 


STEEL  149 

When  carbon  in  steel  is  about  0.5  per  cent  *  or  more 
the  steel  has  the  quality  of  hardening;  that  is,  on  quench- 
ing in  cold  water  from  a  full  red  heat  the  steel  becomes 
intensely  hard  and  quite  brittle;  this  hardness  may  be 
reduced  by  raising  the  steel  to  certain  temperatures  (the 
process  of  tempering)  that  allow  stability  tendencies  to 
approach  equilibrium.  Thus  tools  can  be  made  of  varying 
hardness  suitable  for  cutting  various  materials.  The 
quality  of  hardening  is  intensified  by  increase  in  carbon. 
See  also  page  167. 

While  carbon  is  the  chief  factor  in  the  control  of  the 
physical  properties  of  steel,  these  properties  are  also 
modified  by  other  substances  present. 

Silicon.  —  There  has  been  much  discussion  of  the 
effect  of  silicon  on  steel,  and  the  conclusion  seems  to  be 
that  in  amounts  usually  present  it  is  not  a  seriously  inju- 
rious element  either  to  strength  or  ductility.  It  is  not 
an  important  question,  since  with  present  methods  of 
structural  and  machine  steel  manufacture  silicon  is  usually 
present  only  as  a  trace  or  as  a  maximum  of  0.04  per  cent. 
In  steel  castings,  silicon  is  often  found  up  to  0.4  per  cent 
without  effect  on  ductility,  although  tensile  strength  is 
considerably  increased;  it  is  here,  doubtless,  because  of  its 
capacity  to  reduce  oxides  like  CO,  FeO  and  Fe203,  formed 
during  melting,  and  thus  to  make  the  castings  sound  and 
homogeneous.  In  tool  steel  silicon  is  often  present  up 
to  0.25  per  cent.  But  although  silicon  in  relatively  small 
amounts  in  solution  in  iron  has  small  effect,  with  increas- 
ing amount  the  ductility  is  reduced  with  increase  in  brit- 
tleness.  The  fact  that  ferrite  is  very  ductile  in  mild 
steel,  less  ductile  in  malleable  castings,  and  very  much 

*  The  property  of  hardening  really  appears  when  carbon  equals 
about  0.25  per  cent,  but  the  effect  which  increases  with  the  carbon 
is  not  of  practical  importance  with  carbon  less  than  0.5  per  cent. 


150  MATERIALS  OF   MACHINES 

less  ductile  in  cast  iron  is  probably  due  to  the  increasing 
amount  of  silicon  in  solid  solution. 

Manganese.  —  When  steel  from  the  Bessemer  or  open- 
hearth  process  is  ready  to  pour,  manganese  is  added 
in  the  form  of  ferromanganese  or  spiegeleisen,  and  its 
function,  as  already  explained,  is,  like  silicon,  to  reduce 
CO,  FeO,  Fe2O3  and  to  take  up  free  oxygen  to  form  man- 
ganese oxide.  This  oxide  is  removed  with  the  slag;  but 
a  certain  amount  of  manganese  goes  into  solution  with 
the  iron  and  in  present  practice  in  structural  and  ma- 
chine steels  this  amount  is  from  0.7  to  1  per  cent.  In 
such  amount  the  manganese  prevents  cracking  of  the 
steel  when  it  is  worked  hot.  With  manganese  up  to  0.6 
per  cent  there  seems  to  be  no  effect  on  tensile  strength 
or  ductility;  from  0.6  to  1  per  cent  the  tensile  strength  is 
increased  without  change  of  ductility.  In  tool  steel 
manganese  must  be  kept  low,  because  it  tends  to  cause 
cracking  when  the  steel  is  quenched  in  water  for  purposes 
of  hardening.  If  manganese  in  steel  is  increased  to  2  per 
cent,  the  steel  is  extremely  brittle  and  continues  brittle 
up  to  about  7  per  cent  when,  with  further  increase  of  man- 
ganese, strength  and  ductility  both  increase  until,  at 
14  per  cent  manganese  with  0.85  per  cent  carbon,  tensile 
strength  becomes  about  2.5  times,  and  ductility  about  1.2 
times  that  of  good  mild  steel.  The  material  can  be 
forged  hot,  but  is  so  hard  when  cold  that  it'  can  only  be 
machined  by  grinding.  This  special  manganese  steel  is 
used  for  machine  parts  requiring  great  resistance  to  wear 
and  to  shock. 

Sulphur  makes  steel  "red  short";  that  is,  it  causes 
it  to  crack  when  rolled  hot;  it  also  makes  welding  difficult. 
The  effect  of  sulphur  on  steel  when  cold  has  been  in  doubt, 
because  of  conflicting  evidence.  Of  course,  if  its  produc- 
tion of  brittleness  when  hot  has  resulted  in  the  steel 


STEEL  151 

cooling  with  cracks,  strength  and  general  fitness  for  use 
would  be  less.  If  the  cooled  steel  is  sound  the  effect  of  the 
sulphur  might  depend  on  whether  it  was  present  as  iron 
sulphide  or  manganese  sulphide.  It  seems  probable  that 
the  presence  of  iron  sulphide  would  tend  toward  reduced 
ductility  and  increased  hardness,  and  it  also  seems  prob- 
able that  manganese  sulphide  would  have  less  undesir- 
able effect.  It  certainly  would  be  desirable  to  specify 
sulphur  as  low  as  possible  without  hardship  to  the  steel 
manufacturer. 

Phosphorus  tends  to  cause  the  formation  of  coarse, 
crystalline  structure  during  cooling  of  steel.*  Because  of 
this,  or  for  other  reasons,  the  cooled  steel,  although  its 
static  strength  may  be  somewhat  increased,  yields  more 
easily  to  shocks  and  is  unsafe  as  structural  material,  that 
is,  it  is  "cold  short."  Both  sulphur  and  phosphorus  are 
very  undesirable  in  tool  steel  since  the  tendency  to  crack 
either  hot  or  cold  may  destroy  expensive  tools  during 
or  after  heat  treatment. 

Arsenic  and  copper  are  often  present  in  steel,  but 
with  modern  methods  of  steel-making  the  amount  of 
these  substances  is  almost  always  far  within  the  harmful 
limit. 

Nickel.  —  The  introduction  of  nickel  into  steel  up  to 
about  8  per  cent  increases  the  elastic  limit  and  ultimate 
strength  and  also  slightly  increases  ductility.  Nickel  also 
seems  to  increase  the  hardening  effect  of  a  given  amount 
of  carbon.  Because  of  this,  nickel  steel  is  extensively  used 
as  armor  plate.  In  certain  cases  steel  with  quite  low 
carbon  has  about  3.25  per  cent  nickel  added;  this  gives 
a  ductile  steel  with  a  tensile  strength  of  from  60,000  to 

*  Possibly  this  is  because  phosphorus  causes  extension  of  the  time 
of  cooling  and  thus  gives  opportunity  for  more  complete  crystal- 
lization. 


152  MATERIALS  OF  MACHINES 

80,000  pounds  per  square  inch.  The  armor  plates  made 
from  this  steel  are  "Harveyized  ";  that  is,  they  are  heated 
to  full  redness  for  a  long  period  with  the  outer  surface 
packed  with  carbonaceous  material;  the  steel  absorbs 
carbon,  and  the  surface,  to  the  depth  of  from  1  inch  to 
1J  inches,  becomes  high-carbon  steel,  which  hardens  on 
quenching.  Thus  the  body  of  the  plate  has  the  toughness 
and  strength  —  and  therefore  the  shock-resisting  capacity 
—  of  low-carbon  nickel  steel,  while  the  surface  has  the 
hardness  of  hardened  high-carbon  steel  with  the  intensifi- 
cation of  hardness  due  to  the  presence  of  nickel.  Nickel 
steel  is  often  used  for  structural  and  machine  purposes 
in  places  requiring  great  strength  and  shock  resistance. 
When  welding  is  necessary  nickel  should  be  kept  below 
2  per  cent. 

Tungsten  added  to  steel  renders  it  hard  and  brittle 
and  its  chief  use  is  in  special  steels  for  cutting  tools. 
See  p.  175. 

Chromium.  Ferrochrome  with  wide  variation  of 
chromium  and  carbon  content  is  reduced  from  chrome 
ore  in  the  blast-furnace,  or  in  carbon-lined  crucibles,  by 
strongly  heating  oxide  of  iron  and  oxide  of  chromium 
together.  Chromium  seems  to  unite  with  iron  in  solid 
solution  and  also  in  a  chemical  compound  of  iron,  chro- 
mium and  carbon  that  is  exceedingly  hard.  Chromium 
seems  not  only  to  confer  hardness  of  itself  but  also  to 
intensify  the  hardness  due  to  carbon;  it  also,  as  shown 
by  careful  experiments,*  increases  the  amount  of  carbon 
that  iron  can  hold.  Thus  it  may  cause  a  threefold  in- 
crease in  hardening  of  steel.  Because  of  this  chrome  steel 
with  Cr  about  2  to  2.75  per  cent  and  C  0.9  to  1  per  cent 
is  used  in  projectiles  designed  for  piercing  armor-plates. 

*  See  "Steel  and  Iron  for  Advanced  Students"  by  Hiorns, 
Macmillan,  p.  309. 


STEEL  153 

These  projectiles  require  very  careful  heat  treatment. 
Chrome  steel  is  also  used  for  armor-plate,  for  jaws  of 
crushing  machines  and  for  other  machine  parts  requiring 
great  hardness. 

Vanadium. — Ferrovanadium  is  obtained  from  ores  con- 
taining vanadium  oxide.  This  oxide,  purified  by  chemical 
processes,  is  mixed  with  iron  oxide  and  powdered  pure 
aluminum,  in  a  crucible  with  magnesite  lining,  and  ignited ; 
the  aluminum  combines  with  the  oxygen  of  the  oxides,  and 
the  resulting  iron  and  vanadium,  at  the  high  temperature 
produced  by  combustion  of  the  aluminum,  combine  to 
form  ferrovanadium.  This  is  introduced  into  the  steel 
in  the  ladle  after  treatment  with  spiegel  or  ferromanga- 
nese,  and  any  required  content  of  vanadium  can  thus  be 
obtained. 

Vanadium  has  a  very  powerful  influence  upon  the  phys- 
ical properties  of  steel  either  through  its  direct  influence 
on  the  iron  or  through  its  indirect  influence  upon  the  other 
substances  present.  The  elastic  limit,  ultimate  strength 
and  shock  resistance  of  steel  are  very  greatly  increased 
by  the  presence  of  vanadium  in  amounts  between  0.1 
per  cent  and  0.18  per  cent.  The  steel  usually  contains 
manganese  and  chromium  in  addition  to  the  carbon,  and 
frequently  nickel  also. 

A  standard  vanadium  steel  has  the  following  composi- 
tion: 

Per  cent 

Carbon 0.25  to  0.3 

Manganese 0.5 

Chromium 1.0 

Vanadium.  ,  0.17 


154  MATERIALS  OF   MACHINES 

This  steel  gives  values  about  as  follows: 


Heat  treatment 

Elastic  limit, 
Ib.  per  sq.  in. 

Ultimate 
strength,  Ib. 
per  sq.  in. 

Elongation 
in  2  inches, 
per  cent 

Annealed 

65,000 

90  000 

28 

Oil  tempered  
For  mild  carbon  steel  corre- 
sponding values  

125,000 
38,000 

136,000 
70,000 

18 
32 

Impact  tests  also  show  very  distinct  gain  in  shock 
resistance  due  to  the  presence  of  vanadium. 

Vanadium-nickel  and  vanadium-chrome-nickel  steels 
are  also  used  where  great  strength,  lightness  and  shock 
resistance  are  prime  requirements;  the  latter  steels  have 
a  percentage  range  about  as  follows: 

Per  cent 

Carbon 0.25  toO.45 

Manganese 0.5   to  0.7 

Nickel 1       to  1.5 

Chromium 0.6    to  0.8 

Vanadium about  0.18. 

There  may  be  internal  stresses  in  forged  material, 
similar  to  those  resulting  in  cast  material  from  unequal 
shrinkage.  They  are  usually  the  result  of  working  the 
material  too  cold.  To  illustrate:  when  a  thin  piece  of 
ductile  material  is  laid  on  an  anvil  and  struck  with  a 
hammer,  the  piece  is  made  thinner  and  longer  and  broader. 
Suppose  now  that  the  piece  is  thick  instead  of  thin,  and 
that  it  receives  a  blow  as  before:  the  influence  of  the 
blow  extends  only  a  little  way  into  the  material,  and  the 
surface  is  made  longer  and  broader.  Since  its  extension 
is  resisted  by  the  part  which  is  uninfluenced  by  the  blow, 
the  material  at  the  surface  is  put  in  compression,  and  the 


STEEL  155 

inner  portion  in  tension.  The  initial  part  of  the  stress- 
deformation  diagram  would  be  as  shown  in  Fig.  27.  If 
the  working  is  done  at  a  red  heat,  the  material  is  soft 
and  weak,  and,  therefore,  yields  to  the  stresses  introduced 
by  the  hammering  or  rolling,  and  equilibrium  results. 

Effect  of  lack  of  homogeneousness  of  material  on 
the  stress-deformation  diagram.  —  In  the  manufacture 
of  wrought  iron  the  elements  of  the  piles  of  "  muck-bar  " 
or  scrap  are  drawn  out  in  rolling  into  long  lines  of  crystals, 
which  are  separated  by  more  or  less  slag  or  oxide  of  iron. 
Since  the  pile  may  be  made  up  of  bars  or  scrap  of  entirely 
different  quality,  the  structure  may  lack  homogeneous- 
ness.  This  has  a  tendency  to  modify  the  form  of  stress- 
deformation  diagram.  Suppose,  for  example,  that  a  test 
piece  of  wrought  iron  has  half  of  its  area  of  cross  section 
of  a  material  whose  elastic  limit  is  at  El,  Fig.  30,  and  that 
the  other  half  of  the  cross  section  is  of  material  whose 
elastic  limit  is  at  E.  Let  a  constantly 
increasing  tensile  force  be  applied  to  this 
test  piece.  When  the  stress  reaches  the 
value  represented  by  the  ordinate  E,  the 
weaker  part  of  the  material  begins  to  yield 
more  rapidly  than  the  stronger  part,  and 
the  unit  stress  on  the  stronger  part  is  very 
greatly  increased,  its  elastic  limit  is  ex- 
ceeded, it  also  yields,  and  the  curve  takes 
the  form  shown,  running  nearly  parallel 
to  the  axis  of  X  until  the  stress  is  again  distributed  over 
the  entire  surface  of  the  cross  section;  then  the  curve  rises 
continuously  until  the  maximum  stress  is  reached.  Steel 
may  also  show  this  irregularity,  since  different  parts  of  the 
forging  may  have  different  elastic  limit,  because  of  differ- 
ent heat  treatment,  different  hot  working  or  superficial 
cold  working. 


156  MATERIALS  OF   MACHINES 

Effect  of  cold  working.  —  When  a  piece  of  ductile 
material  is  strained  beyond  its  elastic  limit,  the  character 
of  the  material  is  greatly  changed.  If,  after  a  short 
interval  of  rest,  it  is  tested  again,  its  elastic  limit  and 
elastic  resilience  will  be  found  to  be  higher,  its  tensile 
strength  greater  and  its  ductility  and  ultimate  resilience 
less.  The  stiffness  will  be  but  slightly  changed,  if  at  all. 
By  cold  working,  i.e.,  by  any  means  that  gives  permanent 
set  to  cold  material,  the  elastic  range  is  increased,  the  piece 
is  made  stronger  and  better  able  to  resist  shocks  within 
the  elastic  limit,  but  less  ductile,  and  less  able  to  resist 
shocks  exceeding  the  elastic  limit.  These  changes  are 
shown  graphically  in  Fig.  31.  The  stress-deformation 
diagram  OEABCD  is  such  as  would  usually  result  from  a 
test  of  a  ductile  material,  like  mild  steel  or  wrought  iron. 
On  reaching  some  point,  as  E1}  stress  is  gradually  relieved, 
and  the  curve  descends  to  the  X  axis  at  0\.  On  reappli- 
cation  of  tensile  force  the  curve  rises  along  the  line  QiE1 
nearly  parallel  to  OE.  The  elastic  limit  is  now  at  EI,  a 
point  much  higher  than  the  original  elastic  limit  E.  The 
curve  then  continues,  a  little  higher  than  it  would  if  the 
stress  had  not  been  discontinued,  until  the  maximum 
is  reached  at  H* 

If  the  force  could  have  been  instantly  reapplied  at  Oi, 
the  line  GHJ  would  probably  have  coincided  with  ABC, 
because  the  change  is  a  function  of  the  time  of  resting, 
after  relief  of  stress.  OEABCD  may  be  considered  the 
diagram  of  one  material,  and  OiEiGHJ  the  diagram  of 

*  That  the  maximum  strength  is  increased  has  been  demon- 
strated by  Bauschinger.  He  first  broke  a  long  test  piece  by  tensile 
force.  It  was  of  uniform  cross  section,  and  hence  all  of  its  parts 
must  have  been  strained  well  past  the  elastic  limit.  He  then  broke 
one  of  the  pieces  and  found  increased  strength.  This  was  repeated 
six  times,  and  each  repetition  resulted  in  increased  strength. 


STEEL 

another  material.  It  is  as  if  a  new  test  began  at 
a  represent  the  first  diagram,  and 
6  the  second.  The  elastic  range 
of  6,  represented  by  OiE1}  is 
greater  than  that  of  a,  repre- 
sented by  OE.  The  elastic  resili- 
ence of  6,  represented  by  the  area 
Oi^iFi,  is  greater  than  that  of  a, 
represented  by  OEF.  Experiment 
has  proved  that  the  points  B  and 
C  are  not  changed  in  their  rela- 
tion to  the  axis  of  F  by  the  relief 
of  stress;  and  therefore  the  ductil- 
ity of  a,  represented  by  OD,  is 
greater  than  the  ductility  of  6, 
represented  by  OiD.  The  ulti- 
mate resilience,  proportional  to 
the  total  area  under  the  curve, 
is  evidently  greater  in  a  than  in 
b.  OiEi  is  nearly  parallel  to  OE, 
and  hence  rigidity  is  nearly  the 
same  for  both. 

If,  instead  of  the  almost  im- 
mediate reapplication  of  force,  a 
considerable  interval  of  rest  had 
been  allowed,  say  twenty-four 
hours,  the  elastic  limit  and  ulti- 
mate strength  would  have  been  still 
further  raised,  and  the  diagram 
would  be  like  OiE2LMN.  If  stress 
were  not  discontinued  until  the 
maximum  had  been  nearly  reached, 
the  strained  material  would  resem- 
ble a  very  brittle  material. 


157 

Let 


158  MATERIALS  OF   MACHINES 

It  may  be  stated  as  a  conclusion  warranted  by  exper- 
iment (see  Trans.  Am.  Soc.  Civil  Engineers,  Vol.  XXIV, 
p.  159),  that  stress  of  any  character  beyond  the  elastic 
limit  will  render  a  ductile  material  stronger  and  less  duc- 
tile under  stress  of  any  other  character.  Annealing 
removes  these  effects  almost  completely.  The  process 
of  "cold  rolling,"  by  which  shafting  is  produced,  illus- 
trates the  alterations  of  the  qualities  of  ductile  material 
due  to  stress  beyond  the  elastic  limit.  In  this  process 
iron  is  passed  cold  through  highly  finished  rolls,  under 
intense  pressure.  The  rolled  piece  has  its  length  increased 
and  its  cross  section  reduced,  and  therefore,  since  the 
material  takes  a  "set,"  it  must  be  strained  by  the  treat- 
ment past  its  elastic  limit. 

Professor  Thurston  made  a  series  of  tests  to  determine 
the  effect  of  cold  rolling  upon  iron.  His  experiments 
show  that  there  results  from  the  process:  (a)  an  increase 
in  tensile  strength  of  from  25  to  40  per  cent;  (6)  an  ele- 
vation of  the  elastic  limit  of  from  80  to  125  per  cent; 
(c)  an  increase  of  elastic  resilience  of  from  300  to  400  per 
cent;  (d)  a  decrease  in  ductility  of  about  75  per  cent; 
and  (e)  a  decrease  of  ultimate  resilience  of  about  40  per 
cent.  If,  therefore,  the  product  of  the  process  is  required 
to  withstand  stress  (and  especially  shock),  which  cannot 
exceed  the  elastic  limit,  it  is  far  better  than  the  untreated 
iron;  but  if  there  is  a  possibility  of  shock  exceeding  the 
elastic  limit,  the  unrolled  iron  might  be  better. 

The  process  of  "wire-drawing,"  i.e.,  reducing  the  size 
of  wire  with  increased  length  by  drawing  it  cold  through 
dies,  produces  the  same  result  as  cold  rolling,  the  wire 
requiring  frequent  annealing  to  restore  ductility. 

The  effect  of  repeated  stress.  —  Between  the  years 
1859  and  1870,  A.  Wohler  planned  and  executed  a  series 
of  experiments  for  the  Prussian  Government,  to  determine 


STEEL  159 

the  laws  governing  the  behavior  of  metals  under  repeated 
stress.  By  means  of  his  machines,  forces  of  known  value 
producing  tensile,  compressive,  torsional  or  transverse 
stress  could  be  applied  with  indefinite  repetition,  until 
rupture  occurred,  or  until  it  was  considered  proved  that 
indefinite  repetition  of  stress  could  not  produce  rupture. 
He  formulated  a  law  from  the  experimental  work,  which 
in  substance  is  as  follows: 

Material  may  be  broken  by  repeated  application  of  a 
force  which  would  fail  to  produce  rupture  by  a  single 
application.  The  breaking  is  a  function  of  range  of  stress; 
and  as  the  recurring  stress  increases,  the  range  necessary 
to  produce  rupture  decreases.  If  the  stress  is  reversed, 
the  range  equals  the  sum  of  positive  and  negative 
stress. 

The  experimental  work  of  Wohler  was  amplified  and 
supplemented  by  Professor  Bauschinger  of  Munich.  He 
drew  the  following  conclusions  from  his  experimental 
work: 

(a)  "With  repeated  tensile  stresses,  whose  lower  limit 
was  zero,  and  whose  upper  limit  was  near  the  original 
elastic  limit,  rupture  did  not  occur  with  from  5  to  16 
million    repetitions."     He    cautions    the    designer    that 
this  will  not  hold  for  defective  material,  i.e.,  a  factor  of 
safety  must  still  be  used  for  this  reason;    and  that  the 
elastic  limit  of  the  material  must  be  carefully  determined, 
because  it  may  have  been  artificially  raised  by  cold  work- 
ing, in  which  case  it  does  not  accurately  represent  the 
material.     This  original  elastic  limit  may  be  determined 
by  testing  a  piece  of  the  material  after  careful  anneal- 
ing. 

(b)  "With   often   repeated   stresses,   varying  between 
zero  and  an  upper  stress,  which  is  in  the  neighborhood  of 
or  above  the  original  elastic  limit,  the  latter  is  raised  even 


160  MATERIALS  OF   MACHINES 

above,  often  far  above,  the  upper  limit  of  stress,  and  it 
is  raised  higher  as  the  number  of  repetitions  of  stress 
increases,  without,  however,  a  known  limiting  value  L, 
being  exceeded." 

(c)  "Repeated  stresses,  between  zero  and  an  upper 
limit  below  L,  do  not  cause  rupture;  but  if  the  upper 
limit  is  above  L,  rupture  will  occur  after  a  limited  number 
of  repetitions." 

From  this  it  would  follow  that  keeping  the  range  of 
repeated  stress  within  the  original  elastic  limit  would 
insure  safety  against  rupture  with  any  number  of  repe- 
titions whatever.  But  there  is  a  question  whether  the 
experimenters  have  proved  their  case,  since  they  dealt 
necessarily  with  a  finite  number  of  stress-cycles.  The 
conclusion  that  rupture  with  repeated  stress  is  a  function 
of  range  of  stress  seems  to  be  sound;  but  Professor  Bau- 
schinger's  conclusion  that  there  is  a  limit  of  range  within 
which  there  is  absolute  safety  from  repeated-stress  rup- 
ture seems  questionable.  A  perfectly  elastic  material 
is  one  that  returns  exactly  to  its  initial  condition  after 
deformation  under  stress;  there  is  question  whether  any 
engineering  material  is  perfectly  elastic  for  any  range 
whatever,  even  under  small,  slowly  applied  stress. 

There  is  evidence  *  that  any  change  of  stress  in  iron 
causes  the  magnetic  and  thermo-electric  properties  to 
change  in  an  irreversible  way,  and,  to  quote  Professor 
Ewingif  "Every  variation  leaves  its  mark  on  the  quality 
of  the  piece;  the  actual  quality  at  any  time  is  a  function 
of  all  the  states  of  stress  in  which  the  piece  has  previously 
been  placed.  It  can  scarcely  be  doubted  that  sufficiently 
refined  methods  of  experiment  would  detect  a  similar 
want  of  reversibility  in  the  mechanical  effects  of  stress." 

*  See  papers  by  Prof.  J.  A.  Ewing,  Phil.  Trans.,  1885-1886. 
f  See  his  excellent  book,  "The  Strength  of  Materials,"  p.  55. 


STEEL  161 

Professor  Ewing  also  cites  experiments  by  Lord  Kelvin 
which  show  that*  " repeated  changes  of  stress  have  a 
cumulative  effect  in  reducing  elasticity,  while  Wohler's 
experiments  show  that  they  also  have  a  cumulative  effect 
in  reducing  strength.  It  may  be  conjectured  that  re- 
peated strains  induce  a  change  in  molecular  structure  of 
which  the  fatigue  in  strength  and  the  fatigue  in  elasticity 
are  two  manifestations." 

Annealing  restores  both  original  strength  and  elas- 
ticity; rest  restores  original  elasticity  and,  as  recently 
proved,  f  restores  strength  also. 

In  the  stress-elongation  diagram  of  cast  iron,  Fig.  18, 
stress  was  relieved  at  C  and  the  path  of  diminishing  stress 
is  CDE,  while  the  path  of  reapplied  stress  is  EFC.  These 
two  paths  enclose  an  area  which,  on  this  diagram  of  force- 
space  coordinates,  represents  work.  If  the  up  and  down 
paths  were  identical,  there  would  be  no  enclosed  area  and 
the  work  done  by  the  increasing  force  would  be  completely 
restored  on  relief  of  stress  in  the  same  energy  form,  and 
there  would  remain  in  the  material  no  permanent  result 
of  the  work  done;  the  material  would  be  in  exactly  the 
same  condition  before  and  after  the  application  of  force 
and  hence  would  be  perfectly  elastic.  This  is  what  is 
called  a  reversible  process.  But  if  the  up  and  down 
paths  enclose  an  area,  the  process  is  irreversible,  the 
material  is  not  perfectly  elastic  and  an  amount  of  work 
proportional  to  the  area  fails  of  return  in  its  original 
energy  form;  it  is  converted  into  heat  by  resistance  of 
the  material  to  molecular  change,  the  heat  is  radiated 
away  and  the  resulting  molecular  change  remains.  Repe- 
tition of  this  cycle  —  called  the  hysteresis  cycle  —  does 

*  Swing's  "The  Strength  of  Materials,"  p.  56. 
f  See   "Materials  of   Construction,"  by  Professor    George  B. 
Upton,  John  Wiley  &  Sons. 


162  MATERIALS  OF  MACHINES 

more  internal  work  to  cause  molecular  change,  and  with 
continued  repetition  the  material  would  be  destroyed. 
Wherever  there  is  a  hysteresis  loop  with  repeated  stress, 
the  work  of  destruction  is  under  way.  The  length  of  the 
loop  corresponds  to  the  range  of  stress;  the  width  of 
the  loop  corresponds  to  amplitude  of  molecular  displace- 
ment; and  the  area  of  the  loop,  proportional  to  length 
and  width,  corresponds  to  the  work  done  to  produce  mo- 
lecular change,  and  hence  to  the  destroying  agency. 
Therefore,  increase  in  range  of  repeated  stress  increases 
the  destroying  agency  and  hence  reduces  the  number 
of  cycles  to  cause  fracture.  It  is  found  experimentally 
that  the  width  of  the  loop  increases  with  the  number  of 
cycles,  and  hence  the  destroying  agency  has  an  increas- 
ing value. 

Though  the  ordinary  methods  of  test  do  not  show  a 
hysteresis  cycle  within  the  so-called  elastic  limit  of  steel, 
yet  more  refined  methods  disclose  such  a  loop,*  and  make 
it  extremely  probable  that  repeated  stress,  even  within 
the  original  elastic  limit,  would  cause  rupture  with  a 
sufficient  number  of  repetitions.  Ordinarily  machines 
grow  obsolete  or  wear  out  and  are  discarded  long  before 
fatigue  failure  occurs. 

There  is  another  effect  of  repeated  stress  that  is  inde- 
pendent of  the  effect  on  molecular  structure.  If  very 
minute  flaws  exist  in  the  material,  or  if  continuity  is 
broken  by  small  particles  of  foreign  substance,  the  ten- 
dency of  repeated  stress  is  to  increase  the  size  of  the  im- 
perfections and  a  number  of  these  extending  micro-flaws 
might  join  to  produce  a  large  crack  and  eventually  to 
cause  fracture.  Larger  hidden  flaws  might,  of  course, 
be  extended  similarly  with  the  same  result. 

*  The  phenomenon  known  to  physicists  as  "elastic  after-effects" 
shows  this. 


STEEL  163 

Effect  of  temperature  on  steel.  Many  experiments 
by  various  careful  observers  show  that  when  steel  is  heated 
to  about  500°  F.  its  tensile  strength  begins  to  decrease, 
and  that  at  the  temperature  of  incipient  redness,  about 
1000°  F.,  its  value  is  less  than  half  the  value  at  air  tem- 
perature. It  is  also  known  that  prolonged  and  repeated 
exposure  to  temperatures  of  150°  F.,  or  higher,  produces 
reduction  of  ductility,  so  that  to  insure  safety  periodical 
annealing  is  necessary,  as  in  case  of  chains  of  cranes  for 
lifting  ladles  of  molten  steel  or  hot  ingots. 
^  Factors  of  safety.  —  Machine  stress-members  may 
fail,  not  only  because  of  repeated  stress,  but  also  because 

of: 

(a)  Flaws,  or  other  imperfections  in  the  material; 

(b )  Internal  stresses ; 

(c)  Unhomogeneous material; 

(d)  Shocks; 

(e)  Stresses  which  cannot  be  estimated. 

To  cover  all  these  a  factor  of  safety  is  used;  i.e.,  the 
working  unit  stress  is  equal  to  the  ultimate  unit  strength 
of  the  material,  divided  by  a  number  which  is  called  the 
factor  of  safety. 

Materials  are  so  various  in  their  qualities,  and  the 
conditions  to  which  they  are  subjected  as  machine  stress 
members  are  so  different,  that  it  is  impossible  to  give  any 
value  for  a  factor  of  safety  to  cover  all  cases. 

For  ductile  resilient  material,  like  mild  steel  used  in 
building-frames,  roof -trusses,  bridges,  etc.,  a  low  value 
may  be  used  for  the  factor  of  safety,  because  b,  c  and  d 
given  above  may  be  practically  eliminated  by  proper  speci- 
fications and  careful  inspection,  and  because  the  loads  are 
known. 

But  in  machines  the  conditions  are  dynamic,  and  it  is 
more  difficult  to  estimate  stresses,  especially  when  acci- 


164  MATERIALS  OF   MACHINES 

dental  increases  of  velocity  are  possible,  or  when  lost 
motion,  due  to  wear  or  imperfect  adjustment,  enable 
moving  parts  to  deliver  blows  to  other  parts. 

For  unresilient  or  brittle  materials,  like  cast  iron,  the 
factor  of  safety  needs  to  be  larger,  not  only  because 
of  less  shock-resisting  capacity,  but  because  shrinkage 
stresses  are  always  present  and  there  is,  in  many  cases, 
danger  of  blow  holes  or  spongy  sections.  The  weaken- 
ing effect  of  these  varies  with  the  size  and  form  of  the 
member,  and  with  the  conditions  of  casting.  Hence  the 
factor  of  safety  must  be  determined  in  each  case  by 
the  judgment  of  the  designer. 


CHAPTER  X 
HEAT  TREATMENT   OF   STEEL 

WHEN  molten  steel  cools  slowly  to  air  temperature  its 
structure  is  coarsely  crystalline;  the  size  of  the  crystals 
increases  somewhat  with  the  time  of  cooling  and  with  the 
amount  of  carbon  present.  The  steel  of  a  tool-steel 
ingot  is  coarse,  brittle  and  unfit  for  service.  The  steel  of 
a  mild  steel  ingot  —  though  the  coarse  structure  is  less 
marked  —  also  needs  treatment  to  change  structure  to 
give  required  strength  and  ductility.  The  finest  possible 
structure  of  steel  corresponds  to  highest  strength,  duc- 
tility and  shock  resistance;  this  structure  may  be  pro- 
duced by  heat  treatment,  which  consists  of  heating  and 
cooling  through  certain  temperature  ranges  and  with 
certain  rates  of  temperature  change. 

The  structure  of  steel,  whatever  it  may  be  at  air  tem- 
perature, is  changed  to  the  finest  possible  crystal  size 
when  the  increasing  temperature  reaches  about  1330°  F., 
corresponding  to  the  line  MO,  Fig.  29.  If  the  increase 
in  temperature  continues  from  1330°,  the  crystal  size  grows 
steadily  larger  until  fusion  begins  and  the  size  thus  reached 
is  retained,  whatever  the  method  of  cooling;  if,  after 
reaching  1330°,  the  temperature  is  allowed  to  fall  slowly, 
the  crystal  size  increases  until  the  temperature  of  dis- 
appearing redness  is  reached  and  this  size  is  retained 
independently  of  the  method  of  cooling.  Increase  in 
crystal  size  is  undesirable  when  high  shock  resistance  is 
required,  and,  therefore,  whatever  the  purpose  of  heat 

165 


166  MATERIALS  OF   MACHINES 

treatment  it  should  produce  the  finest  structure  possible 
under  the  circumstances  in  steel  that  is  required  to  resist 
shock. 

When  points  representing  cooling  steel  pass  slowly 
through  the  territory,  Fig.  29,  bounded  above  by  the  line 
KNOJz  and  below  by  the  line  MOJ2,  the  steel  changes 
from  solid  7  (C),  (austenite)  to  eutectic  (a  +Fe3C)o.9c 
mixed,  according  to  the  carbon  content,  with  excess  of 
either  ferrite  (a  iron)  or  cementite  (Fe3C).  Careful 
observation  and  reasoning  by  many  eminent  metal- 
lurgists seem  to  show  that  during  this  change  the  steel 
passes  through  three  intermediate  states  with  varying 
physical  properties.  The  names  given  to  steel  in  these 
successive  states  are  austenite,  the  original  7  (C),  mar- 
tensite,  troostite  and  sorbite. 

Austenite  is  soft  and  ductile,  with  medium  strength 
and  high  shock  resistance. 

Martensite  is  intensely  hard,  strong  under  steady 
stress,  but  with  low  shock  resistance. 

Troostite  has  medium  hardness  and  is  strong,  ductile 
and  tough. 

Sorbite  is  nearly  as  soft  as  austenite  and  is  strong, 
ductile  and  tough. 

With  slow  cooling,  austenite  would  change  through  this 
series  and  become  sorbite  and  the  change  would  be  com- 
plete. But  during  this  change  austenite  does  not  change 
wholly  into  martensite  and  then  wholly  into  troostite; 
the  changes  overlap  so  that  before  all  austenite  has  become 
martensite  the  formation  of  troostite  out  of  martensite 
has  begun  and  all  three  are  present  in  varying  amounts. 
By  the  time  austenite  has  disappeared  the  formation  of 
sorbite  out  of  troostite  has  begun,  and  martensite,  troost- 
ite and  sorbite  are  present  in  varying  amounts;  then 


HEAT  TREATMENT  OF    STEEL  167 

martensite  and  troostite  disappear  in  order,  leaving 
sorbite  alone.  If  the  cooling  could  be  suddenly  checked 
at  any  temperature  and  held  there  to  establish  equilibrium, 
the  steel  could  be  held  in  the  state  corresponding  to  that 
temperature  and  it  would  have  physical  properties  de- 
pending on  the  proportions  of  steel-carbon  forms  present. 
Objects  of  heat  treatment  are  as  follows: 

1.  To  relieve  internal  stress  due  to  cooling  or  mechan- 
ical working  and  to  produce  a  soft  steel  suitable  for  ma- 
chining;  this  is  called  annealing. 

2.  To  restore  fine  grain  to  steel  that  has  been  made 
coarse  by  overheating;    this  is  also  called  annealing,  or 
sometimes  refining. 

3.  To  produce  a  very  hard  steel  for  cutting  edges  of 
tools  or  for  wearing  surfaces;   this  is  called  hardening. 

4.  To  reduce  the  hardness  produced  by  the  hardening 
process  to  any  desired  value  and  at  the  same  time  partially 
to  restore  ductility  and  reduce  brittleness;    this  is  called 
tempering. 

5.  To  render  stress  members  of  machines  tough  and 
shock  resistant  for   severe    service.     This   is   sometimes 
called  toughening. 

6.  To  raise  the  elastic-limit  so  that  in  case  of  springs 
there   may   be   large   yielding   without   permanent    set; 
this  is  called  spring  tempering. 

The  list  will  now  be  considered  in  detail. 

1 .  Annealing.  —  Stresses  due  to  cooling,  or  to  me- 
chanical working  at  too  low  a  temperature,  may  be 
relieved  by  heating  to  about  900°  F.,  a  temperature  just 
below  incipient  redness,  and  cooling  slowly.  The  material 
becomes  soft  enough  at  this  temperature  to  yield  to  in- 
ternal stresses  and  to  take  a  new  adjustment  in  equilib- 
rium. Also,  if  there  is  any  result  of  a  previous  hardening 


168  MATERIALS  OF   MACHINES 

process,  it  is  entirely  removed  before  reaching  900°  F. 
If  very  great  softness  is  required  for  machining,  the  steel 
is  cooled  very  slowly  from  about  1600°  F.  This,  of  course, 
produces  coarse  grain  which  must  be  refined,  if  necessary, 
for  shock  resistance,  by  method  2  or  5. 

2.  Annealing  or  refining.  —  But  if  the  steel  had  been 
cooled  from  some  temperature  above  the  line  MO,  the 
size  of  its  crystal  structure  would  have  been  enlarged 
beyond  the  size  corresponding  to  MO',  the  higher  the 
temperature  from  which  the  cooling  took  place  the  larger 
the  crystal  size.  This  size  of  crystal  with  its  accompany- 
ing brittleness  remains  unchanged  during  cooling  and 
during  reheating  until  MO  is  reached,  when  it  is  changed 
quite  suddenly  and  irresistibly  to  very  fine  crystal  size. 
With  further  increase  of  temperature  the  crystals  grow 
as  before.  In  order  to  fully  restore  the  fine  structure 
from  the  coarse  structure  due  to  overheating^  it  is  necessary 
to  raise  the  temperature  to  the  limit  set  by  the  line  KNO 
so  that  change  to  7  (C)  shall  be  complete.*  Hence  a 
higher  temperature  is  necessary  to  restore  low-carbon 
or  high-carbon  steel  than  to  restore  steel  with  0.9  per  cent 
carbon;  in  fact  with  the  eutectic  proportion,  0.9  per  cent 
carbon,  it  is  only  necessary  to  heat  to  MO.]  After  heat- 
ing the  steel  above  KNOJS,  if  it  is  caused  to  cool  very 
slowly  by  packing  in  sand  or  ashes,  the  change  from  aus- 
tenite  to  sorbite  becomes  complete,  and,  although  the 
crystal  size  increases  during  cooling,  yet  it  is  the  finest 
structure  that  fully  annealed  steel  can  have.  This  steel 

*  It  is  sometimes  necessary  to  repeat  the  reheating  to  insure 
complete  refining. 

t  The  range  of  the  temperature  of  change  during  heating  is 
somewhat  higher  than  during  cooling  and  hence  the  temperature  is 
raised  in  practice  from  80°  to  90°  F.  higher  than  that  corresponding 
to  the  diagram  and  is  held  for  about  fifteen  minutes  at  this  tempera- 
ture to  insure  complete  change  to  y  (C). 


HEAT  TREATMENT  OF    STEEL  169 

is  thus  free  from  internal  stresses,  and  it  is  soft  *  and 
ductile  with  medium  tensile  strength. 

3.  Hardening.  —  Consider    that   steel   with  0.9    per 
cent  C  is  raised  to  a  temperature  of  1400°  F.,  corresponding 
to  h,  Fig.  29.     After  passing  above  0  it  changes  into  y  (C). 
Suppose   now   that   the  steel   is    "  quenched/'    that    is, 
immersed  in  agitated  cold  water.     This  sudden  cooling 
tends  to  check  the  change  of  austenite  through  marten- 
site  and  troostite  to  sorbite.     The  ordinary  methods  of 
quenching  f  cannot  hold  the  steel  in  the  austenite  form, 
and  in  the  quenched  steel  martensite  is  usually  the  pre- 
dominating form  with  a  considerable  amount  of  troostite 
and  very  small  amounts  of  austenite  and  sorbite.     The 
steel,  therefore,  has  the  qualities  of  the  dominant  mar- 
tensite and  is  hard  enough  to  scratch  glass,  and,  although 
statically  strong,  is  brittle.     Steel  fully  hardened  in  this 
way  is  usually  too  hard  and  brittle  for  service  and  must 
be  further  treated  by  the  process  of  tempering. 

4.  Tempering.  —  If  the  hardened  steel  is  raised  in 
temperature  a  faint  yellow  color  appears  at  about  425°  F., 
on  any  portion  of  its  surface  that  has  been  polished,  and 
at  this  temperature  the  tendency  to  change  through  the 
a.-m.-t.-s.t  series  begins  to  be  active  and  the  amount  of 
martensite  decreases  while  troostite  and  sorbite  increase, 
with  corresponding  softening  of  the  steel  and  with  decrease 
in  brittleness. 

At  475°  F.  the  surface  color  of  the  steel  becomes  full 
yellow. 

*  Though  not  with  the  maximum  softness  of  about  1600°  cool- 
ing temperature. 

f  The  rate  of  cooling  depends  not  only  on  the  quantity  and  tem- 
perature of  the  water,  but  also  on  the  relation  of  cooling  surface  to 
volume  of  the  steel;  hence  thick  bars  would  cool  more  slowly  than 
thin  bars. 

t    Austenite-martensite-troostite-sorbite  series. 


170  MATERIALS  OF   MACHINES 

At  540°  F.  the  surface  color  of  the  steel  becomes  purple. 

At  560°  F.  the  surface  color  of  the  steel  becomes  bright 
blue. 

At  600°  F.  the  surface  color  of  the  steel  becomes  dark 
blue. 

With  these  increases  in  temperature  there  is  accompany- 
ing increase  of  power  of  the  steel  to  change  through  the 
a.-m.-t.-s.  series  toward  sorbite,  and  if  the  steel  is  held 
for  a  few  minutes  at  a  temperature  corresponding  to  any 
one  of  these  colors,  there  will  be  a  reduction  in  hardness 
which  corresponds  to  the  temperature. 

5.  Toughening.  —  When  steel  is  annealed  by  heating 
to  a  temperature  slightly  above  KNOJS  and  cooling  very 
slowly  to  air  temperature  the  crystal  size  increases  through- 
out the  range  down  to  about  1000°  F.  and  with  this  in- 
crease there  is  loss  of  ductility  and  shock  resistance.  Let 
the  temperature  above  KNOJS  that  gives  full  transfor- 
mation into  7  (C)  be  called  W,  and  the  temperature  of 
incipient  redness,  about  1000°  F.,  be  called  V.  The 
increase  in  crystal  size  and  in  brittleness  occurs  between 
W  and  V'}  the  change  from  austenite  to  sorbite  with  re- 
sulting softness  and  ductility  may  occur  entirely  below  V. 
Therefore  if  steel  is  heated  to  W  and  quenched  to  V,  the 
increase  in  crystal  size  will  be  suppressed,  or  at  least 
greatly  diminished;  and  if  the  steel  is  allowed  to  cool 
slowly  from  V  to  air  temperature  the  a.-m.-t.-s.  series  can 
be  completed  during  the  cooling.  Both  the  holding  of  fine 
grain  and  the  completion  of  the  a.-m.-t.-s.  series  increase 
strength  with  nearly  constant  ductility  and  hence  this 
double  process  toughens  the  steel.  This  process  was 
first  conceived  by  Mr.  John  Coffin  and  applied  to  car 
axles  at  the  Cambria  Steel  Works  at  Johnstown,  Pa., 
with  remarkable  results.  It  is  now  very  generally  used 


HEAT  TREATMENT  OF  STEEL  171 

by  manufacturers  for  steel  that  is  to  be  subjected  to  severe 
and  repeated  shocks. 

6.  Spring  tempering.  —  When  steel  is  heated  to  W  and 
quenched  in  a  medium  like  oil,  which  cools  it  less  rapidly 
than  water,  the  a.-m.-t.-s.  series  is  more  nearly  complete 
because  of  longer  time,  and  the  steel  is  less  hard  and  more 
ductile;  but  the  martensite  is  still  a  potent  factor  in 
determining  the  physical  properties  of  the  steel  and  it 
not  only  hardens  and  reduces  ductility,  but  it  also  raises 
the  elastic  limit  and  thus  gives  a  wider  range  of  yielding 
within  the  elastic  limit.  By  regulation  of  the  time  of 
quenching  and  sometimes  by  subsequent  tempering,  it  is 
possible  to  control  the  steel-carbon  states  so  as  to  give 
the  required  elastic  range  for  all  kinds  of  spring  service 
and  also  to  hold  brittleness  within  necessary  limits. 

Hot  working  of  steel.  —  When  steel  is  heated  so  that 
its  representative  point  on  the  equilibrium  diagram, 
Fig.  29,  is  in  the  territory  above  KNOJz,  the  structure 
is  coarse,  the  size  of  grain  being  nearly  proportional  to  the 
temperature  above  MOJ2,  and  the  grain  size  persists 
during  cooling  to  air  temperature;  hence  steel  cooled 
from  high  temperatures  is  coarse-grained  and  brittle. 
But  if  steel  at  these  high  temperatures  is  mechanically 
worked  so  that  its  dimensions  are  changed,  as  by  rolling 
or  hammering,  the  coarse  crystals  are  broken  up  and  the 
grain  becomes  fine.  If  the  mechanical  working  ceases 
while  the  steel  is  still  at  high  temperature,  the  crystals 
increase  again  to  the  size  corresponding  to  the  temperature. 
If,  however,  the  mechanical  working  continues  until  the 
temperature  is  reduced  to  about  900°  F.,  "black  heat," 
the  steel  will  have  a  very  fine  grain  with  corresponding 
physical  properties.  The  coarsely  crystalline  ingots  from 
the  crucible  steel  process  are  heated  and  hammered  into 
commercial  bars,  and  the  hammering  continues,  the 


172  MATERIALS  OF  MACHINES 

intensity  of  blows  decreasing  with  temperature,  until  black 
heat  is  reached,  and  the  steel  is  thus  given  very  fine  grain. 

Annealing  forgings.  —  In  complex  forgings,  however, 
it  is  impossible  to  work  all  parts  uniformly  from  forging 
heat  to  black  heat;  therefore  some  parts  of  the  forging 
will  cool  from  high  temperature  without  working  and  hence 
with  coarse  grain,  while  other  parts  will  have  the  fine 
grain  due  to  careful  working.  This  forging  can  be  given 
a  uniform  grain  by  heating  uniformly  to  the  temperature 
W  and  cooling  very  slowly.  This  uniform  grain,  however, 
will  not  have  minimum  size  since  it  has  had  opportunity 
to  grow  through  the  temperature  range  W  to  V.  The 
ideal  treatment  for  this  forging  if  it  is  to  endure  severe 
and  repeated  shock  is  to  heat  uniformly  to  W,  to  quench 
as  suddenly  as  possible  *  to  V,  and  then  allow  slow  cooling 
to  air  temperature.  This  insures  fine  grain  and  comple- 
tion of  the  a.-m.-t.-s.  series,  thus  giving  a  maximum  of 
toughness  and  shock  resistance. 

Annealing  steel  castings.  —  Since  steel  castings  cool 
from  a  molten  state,  and  since  the  cooling  must  be  rela- 
tively slow,  it  follows  that  they  must  have  a  coarse  grain. 
Moreover  shrinkage  stresses  are  greater  in  steel  castings 
than  in  cast  iron  because  of  the  higher  casting  temperature, 
and  hence  steel  castings  should  be  annealed  to  relieve 
stress  and  to  refine  grain  if  the  best  results  are  to  be  pro- 
duced in  resisting  stress  and  shock.  Either  the  annealing 
or  toughening  process  may  be  used,  but  the  latter  will 
produce  better  results. 

Case-hardening.  —  Many  steel  forms  require  a  very 
hard  surface  to  resist  wear  or  impact,  and  a  tough  core  to 
resist  fracture.  Forms  of  low-carbon  steel  having  the 
required  core  properties  may  be  case-hardened  for  such 

*  Complex  forgings  may  require  a  slower  cooling  medium  than 
water,  like  oil  or  moving  air,  to  prevent  over-stress. 


HEAT  TREATMENT  OF  STEEL       173 

purposes.  The  forms  are  packed  in  carbonaceous  mate- 
rial, like  wood  or  bone-charcoal,  in  boxes  from  which  air 
is  excluded,  and  the  temperature  is  raised  to  full  redness, 
and  maintained  for  a  sufficient  time  to  produce  the  de- 
sired result.  Carbon  migrates  into  the  steel  and  goes 
into  solid  solution  at  this  temperature.  The  depth  of  the 
effect  and  the  percentage  of  carbon  depends  on  the  tem- 
perature and  the  time  of  exposure.  The  surface  is  thus 
converted  into  high-carbon  steel  while  the  core  remains 
unchanged.  The  steel  form  quenched  from  the  case- 
hardening  process  has  a  hardened  surface,  but  since  the 
carbonizing  temperature  is  higher  than  is  needed  for  the 
hardening,  the  grain  will  be  coarser  than  is  desirable 
and  it  would  be  better  to  cool  slowly,  reheat  to  W  and 
quench,  thus  getting  the  desired  hardness  with  fine  grain 
and  toughness.  Case-hardened  steel  pieces  may  be  heat 
treated  exactly  like  high-carbon  steel;  they  may  be 
annealed,  hardened  and  tempered. 

The  process  of  "Harveyizing  "  is  really  case-hardening 
applied  to  large  armor-plates. 

Mild  steel  is  often  converted  into  high-carbon  steel  at 
its  surface  by  immersion  for  some  time  in  a  molten  bath 
of  potassium  cyanide,  KCN,  which  yields  up  its  carbon 
to  the  steel. 

High-speed  tool  steels.  —  The  output  of  a  cutting- 
tool  of  carbon  steel  is  limited  because  of  the  limit  to  cutting 
speed.  The  work  done  in  removing  metal  as  chips  is 
practically  all  transformed  into  heat  which  raises  the 
temperature  of  the  tool,  of  the  chips  and  of  the  piece 
from  which  the  chips  are  cut.  This  heat  is  radiated  away 
and  when  equilibrium  is  established  between  heat  devel- 
opment and  heat  radiation  the  cutting  tool  will  have  a 
definite  temperature.  If  the  cutting  speed  is  increased, 
the  work  —  and  hence  the  heat  developed  —  is  increased 


174  MATERIALS  OF   MACHINES 

proportionately,  and  the  temperature  of  the  cutting  tool, 
etc.,  must  rise  in  order  that  radiation  shall  increase  to  dis- 
pose of  the  increased  heat  developed.  With  a  given  mate- 
rial to  cut,  and  with  given  conditions  of  feed  and  depth 
of  cut,  there  is,  therefore,  a  definite  relation  between  the 
cutting  speed  and  the  temperature  of  cutting  tool;  hence 
the  output  of  a  cutting  tool  is  a  function  of  the  tempera- 
ture that  it  can  endure  safely. 

If  the  temperature  of  a  hardened,  tempered  carbon- 
steel  tool  is  raised  in  service  above  that  at  which  it  was 
tempered  the  temper  will  be  further  drawn,  the  edge  will 
be  softened  and  will  fail.  The  limits  of  temperature  for 
such  tools  is  about  450°  F.  Modern  high-speed  tool  steels 
hold  an  edge  satisfactorily  at  red  heat. 

About  1860-70  Robert  Mushet  of  the  Titanic  Steel 
Company  in  England  discovered  that  if  steel  contained 
tungsten,  chromium  and  manganese,  together  with  high 
carbon  and  the  usual  other  substances,  this  alloy  copied 
slowly  in  £,ir  was  nearly  as  hard  as  carbon  steel  quenched 
in  water.  This  "  Mushet  "  steel  or  self-hardening  steel 
or  air-hardening  steel,  as  it  was  variously  called,  was 
used  for  many  years  because  it  increased  the  possible 
cutting  speed  —  since  its  temper  could  not  be  drawn. 

Messrs.  Frederick  W.  Taylor  and  Maunsel  White  of  the 
Bethlehem  Steel  Works  carried  out  a  masterly  series  of 
investigations  *  which  led  them  to  the  invention  of 
modern  high-speed  steels  and  which  by  increasing  the 
cutting  efficiency  of  tool  steel  from  100  to  200  per  cent 
has  revolutionized  machine  shop  practice. 

The  development  of  these  steels  can  be  shown  best  by 
reproducing  a  table  from  Mr.  Taylor's  article.  This 
table  gives  the  composition  and  cutting  speed  of  four 

*  See  Transactions  of  Am.  Soc.  Mech'l  Eng's,  Vol.  XXVIII,  on 
"The  Art  of  Cutting  Metals,"  by  F.  W.  Taylor,  p.  31. 


HEAT  TREATMENT  OF    STEEL 


175 


steels  that   are  representative,  as  Mr.  Taylor   says,  of 
four  eras  in  the  development  of  metal  cutting  tools. 

The  first  is  Jessop  steel  which  may  represent  the  era 
of  carbon-steel  tools;  the  second  is  Mushet  steel,  the 
first  of  the  self-hardening  steels;  the  third  is  the  original 
Taylor-White  steel,  the  first  of  the  high-speed  steels; 
while  the  fourth  was  the  best  modern  high-speed  steel  in 
1906  when  Mr.  Taylor's  article  was  written. 


. 

. 

"S 

8 

- 

. 

"S 
8 

» 

4d 

!l8 

o>  "S 

3  a 

(_i 

d  "2 

d"fl 

fe 

0^ 

| 

83^ 

Kind  of  steel 

I1 

S8 

I1 

a 

| 
^ 

££ 

M  ,_, 

|a 

Vanadi 
perce 

a 
1 

h 

PH 

^3 

a 
3 

peed  in  f 
ninute  c 
medium 

O 

CO 

CO 

1  047 

0  189 

0  206 

0  017 

0  017 

16 

Mushet  

5.44 

0.398 

2.15 

1.578 

1.044 

26 

Original  Taylor-  White 

8 

3.8 

1.85 

0.3 

0.15 

0.025 

0.03 

58  to  61 

Best  modern  high 

speed  1906  

18.91 

5.47 

0.67 

0.11 

0.29 

0.043 

99 

Thus  the  cutting  speed  was  increased  over  sixfold. 
Progress  from  Mushet  steel  to  best  modern  high-speed 
steel  shows  very  great  increase  in  tungsten  and  chromium, 
very  great  decrease  in  carbon,  manganese  and  silicon,  and 
the  introduction  of  the  new  element  vanadium. 

Study  of  the  high-speed  steels  by  the  equilibrium  dia- 
gram is  beyond  the  scope  of  this  book,  but  the  heat 
treatment  necessary  for  best  results  is  as  follows:  * 

The  cutting  end  of  the  tool  is  first  raised  slowly  and 
uniformly  to  a  bright  cherry-red;  then  it  is  raised  as 
rapidly  as  possible  to  a  temperature  at  which  the  edges 
of  the  tool  begin  to  fuse;  the  whole  end  of  the  tool  must 
be  raised  uniformly  to  this  temperature  of  incipient  fusion. 


*  From  Mr.  Taylor's  paper. 


176  MATERIALS  OF   MACHINES 

The  tool  is  then  plunged  into  a  molten  lead  bath  at  a  tem- 
perature of  1150°  F.,  where  the  tool  temperature  is  very 
rapidly  reduced  to,  or  below,  1550°  F.  The  amount  of 
lead  in  the  bath  must  be  enough  so  that  its  temperature 
shall  not  be  sensibly  raised  by  the  heat  given  out  by  the 
cooling  tool,  because  it  is  important  that  the  temperature 
of  the  tool  shall  not  be  raised  at  all,  at  any  time  during 
this' cooling  process.  From  the  temperature  of  1550°  F., 
or  lower,  the  cooling  to  air  temperature  may  be  fast  or 
slow  without  harmful  result.  The  process  thus  far  is 
called  the  "  high  "  heat  treatment. 

The  tool  is  then  given  "low  "  heat  treatment  as  follows: 
it  is  heated,  slowly  at  first,  and  then,  through  the  agency 
of  a  lead  bath  which  is  kept  at  about  1150°  F.,  to  a  tem- 
perature that  must  be  above  700°  F.,  and  below  1240°  F.; 
it  is  held  at  this  temperature  about  five  minutes  and  then 
cooled  either  in  an  air-blast  or  simply  by  exposure  to  still 
air.  The  tool  is  then  ready  for  use  and  it  will  hold  its 
cutting  edge  after  it  has  grown  red  hot  under  the  cut. 
It  is  important  that  the  temperature  shall  not  exceed 
1240°  F.  during  this  low  heat  treatment  because  there 
would  result  great  reduction  of  the  quality  of  "red- 
hardness." 


CHAPTER  XI 
NON-FERROUS  ALLOYS 

WHEN  two  metals  are  melted,  together,  one  usually 
takes  the  other  into  liquid  solution,  or  perhaps  the  two 
metals  take  each  other  mutually  into  solution,  with  wide 
range  of  composition.  The  temperature  of  solidification 
of  the  preponderating  metal  is  sometimes  reduced  by  the 
introduction  of  the  other  metal,  as  when  an  increasing 
amount  of  zinc  is  added  to  copper;  or  the  solidification 
temperature  may  be  increased,  as  when  the  process  is 
reversed  and  increasing  amounts  of  copper  are  added  to 
zinc.  When  the  cooling  alloy  solidifies,  the  solution  may 
continue  into  the  solid  state;  or  the  alloy  constituents 
may  separate  completely  from  each  other;  or  chemical 
combination  of  portions  of  the  constituents  may  occur; 
or  there  may  be  combinations  of  these  results.  In  most 
of  the  alloys  that  are  useful  to  the  engineer,  the  solid  is 
composed  of  one  or  more  crystallized  solutions.  In  some 
cases  there  are  several  different  possible  solutions,  and 
the  ones  that  form  depend  on  the  proportions  of  the  con- 
stituents present. 

Alloys  with  copper  as  chief  constituent  are  most  im- 
portant in  engineering  work. 

The  copper-zinc  alloys  are  usually  called  brass. 

In  the  upper  part  of  Fig.  32,  the  brass  equilibrium 
diagram  *  is  shown  with  the  entire  range  from  copper 

*  This  diagram  is  from  a  paper  on  the  Constitution  of  the  Copper 
Zinc  Alloys  by  Mr.  E.  M.  Shepherd  in  the  Journal  of  Physical  Chem- 
istry, Vol.  VIII,  p.  421. 

177 


178 


MATERIALS  OF   MACHINES 


100  per  cent  to  zinc  100  per  cent.     In  the  lower  part  of 
Fig.  32  are  curves  that  show  strength  and  ductility  of  the 


§30,F- 


•A.R.C.  Alloys  Research  Committee  of  the  British  Institution  of  Mechanical  Engineers. 

FIG.  32. 


NON-FERROUS  ALLOYS  179 

alloys  of  varying  composition  in  the  forms  of  castings, 
worked  rods  and  annealed  alloy. 

In  the  equilibrium  diagram  the  curve  of  incipient  solidi- 
fication consists  of  six  branches,  and  there  are  six  corre- 
sponding solid  solutions  or  phases  of  copper  and  zinc.  The 
branches  and  corresponding  phases  are  as  follows: 

Branch  Phase 

AB a. 

BC ft 

CD y 

DE 5 

EF € 

FG y 

A  liquid  solution  of  zinc  in  copper,  represented  by  the 
point  h  on  the  diagram,  may  be  followed  in  cooling.  On 
reaching  hi  the  solution  begins  to  solidify,  beginning  with 
the  formation  of  crystals  low  in  zinc  and  continuing  with 
the  formation  of  crystals  with  steadily  increasing  zinc  con- 
tent until  h%  is  reached  and  solidification  is  complete.  With 
sufficiently  slow  cooling  diffusion  probably  would  produce 
a  homogeneous  solid  solution  of  a  crystals.  The  result 
would  be  the  same  anywhere  from  A  and  62  (copper  100 
per  cent  to  70  per  cent)  except  that  the  proportion  of  zinc 
would  steadily  increase  and  color  would  change  grad- 
ually from  copper-red  to  light  yellow  at  about  90  per  cent 
copper,  and  to  dark  yellow  at  62  with  copper  70  per  cent. 

Reference  to  the  lower  curves  shows  that  addition  of 
zinc  to  copper  up  to  30  per  cent  Zn,  producing  the  a  solid 
solution,  gives  increase  in  strength  and  in  ductility,  and 
hence  in  shock  resistance,  whether  the  resulting  brass 
is  in  the  form  of  castings,  annealed  brass  or  worked  rods. 
Also  any  solution  between  65  and  63  (copper  70  per  cent 
and  64  per  cent)  in  cooling  eventually  enters  the  field 
where  a.  is  the  stable  solid  solution,  and  this  range  is  char- 
acterized by  high  strength  and  ductility  —  and  hence 


180  MATERIALS  OF  MACHINES 

high  shock  resistance.  Between  63  and  Cs  (copper  64 
per  cent  to  40  per  cent)  equilibrium  corresponds  to  a 
mixture  of  a  and  7  solid  solutions,  and  through  this  range 
there  is  first  a  steep  rise  in  strength  up  to  about  53  per 
cent  copper  and  a  steep  drop  in  ductility  to  zero  at  copper 
about  50  per  cent,  and  then  the  strength  falls  very  rapidly. 
This  change  is  probably  due  first  to  the  influence  of  in- 
creasing a  +  7  eutectic  and  later  to  increasing  amount 
of  free  7  solution.  From  50  per  cent  copper  to  the  pure 
zinc  end  of  the  range  the  alloys  are  worthless  to  the  en- 
gineer. 

Copper-zinc  within  the  range  (copper  63  per  cent  to 
56  per  cent)  is  sometimes  called  Muntz  metal;  this  alloy 
can  be  rolled  or  forged  at  a  red  heat,  that  is,  in  the  field 
BCzCi,  where  it  is  in  the  form  of  0  solution;  it  is  used 
for  sheathing  and  fastenings  for  ships.  The  lower  dia- 
gram shows  that  with  this  alloy  there  is  some  sacrifice  of 
ductility  to  gain  strength. 

For  castings  the  alloy  usually  contains  copper  about 
67  per  cent,  while  for  ingots  to  be  drawn  into  tubes  or 
wire  the  copper  is  about  70  per  cent,  corresponding  nearly 
to  maximum  ductility. 

Cold  working  breaks  down  the  crystalline  structure  of 
brass,  increasing  strength  and  brittleness,  and  during  the 
processes  of  drawing  tubes  or  wire  the  brass  must  be  fre- 
quently annealed  to  restore  ductility.  The  effect  of 
cold  working  is  obvious  on  comparison  of  the  tensile 
strength  curve  of  worked  rods  with  the  curve  of  annealed 
brass  in  Fig.  32.  This  increase  in  strength  must  have 
been  accompanied  by  reduction  of  ductility. 

The  annealing  is  accomplished  by  heating  the  brass  to  a 
temperature  between  1100°  F.  and  1200°  F.  where  recrys- 
tallization  occurs.  Quenching  from  this  temperature 
tends  to  hold  crystals  small  whereas  they  increase  in  size 


NON-FERROUS  ALLOYS  181 

with  slow  cooling.  With  either  method  of  cooling  the 
alloy  is  soft. 

The  quality  of  brass  is  often  affected  by  the  presence  of 
substances  other  than  copper  and  zinc;  these  substances 
may  enter  as  impurities  with  one  of  the  constituents,  or 
they  may  be  purposely  introduced  because  of  their  desir- 
able influence  of  physical  properties. 

Aluminum  in  brass.  —  In  experiments  recorded  in  a 
book  on  "Alloys,"  by  Sexton,*  page  108,  aluminum,  0  to 
5  per  cent,  was  added  to  brass  with  60  per  cent  copper,  and 
to  brass  with  70  per  cent  copper.  The  copper  content 
was  kept  constant  and  the  100  per  cent  was  made  up  by 
aluminum  and  zinc.  In  other  words,  aluminum  displaced 
zinc  up  to  5  per  cent.  In  both  cases  the  result  was  an 
increase  in  tensile  strength  and  a  reduction  in  ductility. 
Since  this  same  result  could  be  accomplished  more  cheaply 
by  increasing  the  proportion  of  zinc,  the  use  of  aluminum 
would  seem  hardly  to  be  justified.  Of  course  a  small 
amount  of  aluminum  would  be  useful  as  a  flux  in  melting 
and  casting,  reducing  copper  oxide  and  removing  gas  that 
would  produce  porosity;  but  this  aluminum  would  not 
appear  in  the  alloy. 

Manganese  in  brass.  —  When  manganese,  0  to  7  per 
cent,  displaces  zinc  in  brass  with  60  per  cent  copper  t  the 
effect  is  slight  increase  in  strength  and  slight  decrease  in 
ductility.  Also  there  is  the  same  result  when  manganese, 
0  to  10  per  cent,  displaces  zinc  in  brass  with  70  per  cent 
copper.  The  result  here,  as  in  the  case  of  aluminum, 
would  not  seem  to  justify  the  use  of  manganese.  The 
manganese,  like  the  aluminum,  may  be  used  for  a  flux. 

Iron  in  brass.  —  Iron  is  often  present  in  brass  in  very 
small  amount,  derived  from  iron  tools  used  during  melting 

*  Scientific  Publishing  Company,  Manchester, 
f  "Alloys,"  by  Sexton,  p.  117. 


182  MATERIALS  OF   MACHINES 

and  cooling.  Iron  is  also  introduced  into  brass  up  to  a 
little  more  than  1  per  cent  giving  a  ternary  alloy  called 
"  delta  metal  "  which  is  said  to  have  advantages  of 
strength  and  ductility.  The  iron  is  first  alloyed  with  the 
zinc,  and  this  alloy  is  then  melted  with  copper. 

Arsenic  and  antimony  in  very  small  quantities  are 
often  present  in  brass,  being  brought  as  impurities  in  the 
copper;  both  are  very  undesirable,  making  the  brass  hard 
and  brittle.  Antimony  is  especially  bad  in  brass  that  is 
to  be  rolled  or  drawn  since  it  produces  "  cold-shortness  "; 
it  should  not  exceed  0.01  per  cent.  The  ill-effect  of 
arsenic  is  less,  but  it  should  not  exceed  0.05  per  cent. 

Oxygen  in  brass.  —  The  copper  constituent  of  brass 
oxidizes  very  readily  during  melting  and  casting,  and 
copper  oxide  formed  makes  the  brass  weak  and  brittle. 
As  stated  above  the  copper  oxide  can  be  reduced  by  use 
of  aluminum  or  manganese  used  as  a  flux;  phosphorus  also 
is  often  used  very  effectively  for  this  purpose. 

Bronzes  or  copper-tin  alloys.  —  Fig.  33  gives  the 
equilibrium  diagram  *  for  the  copper-tin  alloys,  with 
the  tensile  strength  and  ductility  diagram  in  the  same 
relation  as  for  copper-zinc  in  Fig.  32.  The  strength  and 
ductility  curves  show  that  useful  alloys  for  stress-members 
are  confined  to  the  range  copper  100  per  cent  to  copper 
about  85  per  cent. 

The  equilibrium  diagram  shows  that  several  solid 
solutions  and  one  chemical  compound  are  formed  in  the 
series;  the  a  solution  seems  to  possess  fair  strength  and 
ductility  and  hence  shock  resistance;  the  /3  solution 
obviously  is  not  stable  at  air  temperature,  and  the  presence 
of  the  free  d  solution  is  fatal  to  ductility.  With  ordinary 

*  From  a  paper  "  The  Constitution  of  the  Copper-Tin  Alloys," 
by  E.  S.  Shepherd  and  E.  Blough,  Journal  of  Physical  Chemistry, 
Vol.  X,  p.  630. 


NON-FERROUS  ALLOYS 


183 


rate  of  cooling  /3  crystals  in  small  amount  are  retained 
with  copper  under  91  per  cent  and  a  larger  amount  may 
be  held  by  quenching. 

The  copper-tin  alloy  that  gives  the  best  combination 
of  strength  and  ductility,  and  hence  of  shock  resistance, 
is  copper  90,  tin  10,  and  this  alloy  is  most  commonly  used 
in  machines. 


ait  one  Thousand.Pounds  per  Square  Inch  GO  Q  L4emP-  ^  dn'  £  g  »c 

DSgg&ss.-olis  s  ^  i  £ 

^ 

EQUIL 

1BRIUN 
FROM 

1   DIAG 
SHEPI 

MM  OF  COPPER  AN 
HERD  AND  BLOUGH. 

D  TIN 

=.  S  8  8  S 
Ductility  in_^  Elongation 

V 

liquioN 

N^., 

quid 

\ 

a* 

V 

*-^, 

^s» 

& 

%^ 

^x 

i:*"^ 

a 

—  V 

^ 

P 

7-Cug 

Sn 
Cu3Sn 

*  liquid 

~~^ 

\ 

^,5" 

S 

a+^ 

5 

t 
g 

Cu 

M 

..«, 

uid 

\ 

'- 

6 

X)       i 

0 

g 

0         7 

0 

e 

)         5 

)         4 

0         3 

0         2 

0         1 

0          ( 

1JO         2b         30 
TENSILE  STRENGTH 

4JO         5JO         ^0         7 
AND  DUCTILITY,  COP 

0         8 
3ER-TI 

0         9b        1( 
M  ALLOY. 

L 

y-Tens 

lc 

Strength,  Shepherd-U 
Tested  as  Oast. 

3ton 

°7 

/ 

1 

/ 

/ 

LD» 

ctility 

,fc 

hepherc 
quer 

-Upton, 
ched  in 

Heated  1 
water. 

o  Rednc 

ss  and 

v 

sS*f 

.ctility,  ' 
T« 

ihepherc 
ted  as  C 

-Upton 
ist. 

V 

V 

\ 

D 

uctility, 
Tested 

Thuretoi 
is  Cast. 

7 

\  \ 
\  ^N 
\ 
\ 

\ 
\ 

^,s' 

/ 

FIG.  33. 


184  MATERIALS  OF   MACHINES 

Heat  treatment  of  bronzes.  —  Shepherd  and  Upton 
made  a  careful  series  of  tests  of  the  physical  qualities  of 
the  copper-tin  alloys.*  The  strength  and  ductility 
curves  of  Fig.  33  are  taken  from  their  report.  Their 
work  also  included  a  study  of  heat  treatment  of  these 
alloys.  The  heat  treatment  methods  were  as  follows: 

A.  —  Heated  to  low-red,  water  quenched. 

B.  —  Held  one  week  at  1000°  F.,  water  quenched. 

C.  —  Tested  as  cast. 

D.  —  Held  one  week  at  752°  F.,  furnace  cooled. 

The  lettered  curves  of  Fig.  34  show  the  effect  of  these 
methods  of  heat  treatment  upon  the  ultimate  strength 
of  bronzes  from  copper  100  per  cent  to  copper  65  per  cent. 
In  Fig.  35  the  curves  show  the  corresponding  effects  upon 
ductility.  Methods  B  and  D  are  of  scientific  interest, 
but  only  methods  A  and  C  can  be  practically  applied. 
Curves  A  and  C,  Fig.  34,  show  that  heating  castings  to 
redness  and  quenching  has  little  effect  on  strength  through 
the  range  copper  100  per  cent  to  87  per  cent;  but  that 
from  copper  87  per  cent  to  78  per  cent,  the  heat  treatment 
causes  a  distinct  increase  in  strength.  Referring  to  the 
equilibrium  diagram  Fig.  33  shows  that  alloys  in  the  a 
field  are  unaffected  by  quenching,  whereas  raising  the 
alloys  into  the  a  +  /3  field  and  quenching  through  the 
a.  +  8  field  increases  strength.  This  result  is  probably 
due  to  control  of  proportions  of  a,  0  and  5. 

Fig.  35  shows  that  quenching  increases  ductility  through- 
out the  entire  range  copper  100  per  cent  to  77  per  cent. 
This  result  is  probably  due  to  the  holding  of  fine  grain 
by  quick  cooling. 

*  See  Journal  of  Physical  Chemistry,  Vol.  IX,  No.  6,  p.  441, 
June,  1905. 


NON-FERROUS   ALLOYS 


185 


\ 


\ 


100 


(to 


85  80 

Per  cent.  Copper 

FlG.  34. 


TO 


95 


90  85  80 

Per  cent  Copper 

FIG.  35. 

A.  —  Heated  to  low-red,  water  quenched. 

B.  — Held  one  week  at  1000°  F.,  water  quenched. 

C.  — Tested  as  cast. 

D.  —  Held  one  week  at  752°  F.,  furnace  cooled. 


75 


186  MATERIALS  OF  MACHINES 

M.  Guillet's  conclusions  *  from  his  experiments  on 
heat  treatment  of  bronzes  are  as  follows: 

1.  In  the  case  of  alloys  containing  over  92  per  cent  of 
copper,  the  tenacity  is  slightly  increased  by  quenching 
between  752°  F.  and  1112°  F.,  and  the  elongation  is  sim- 
ilarly affected. 

2.  In  the  case  of  alloys  containing  less  than  92  per  cent 
of  copper  the  tenacity  and  the  elongation  increase  de- 
cidedly as  soon  as  the  quenching  temperature  exceeds 
932°  F. 

3.  Maximum  strength  is  reached,  whatever  the  com- 
position of  the  alloy,  at  a  quenching  temperature  of  about 
1112°  F. 

4.  Maximum  elongation  is  reached  by  quenching  from 
temperatures  which  vary  with  the  composition  of  the  alloy. 
With    91    per   cent  copper,  maximum  elongation  corre- 
sponds  to   a  quenching  temperature  of    1472°  F.,   while 
with  79  per  cent,  the  maximum  elongation  corresponds 
to  a  quenching  temperature  of  1112°  F. 

•5.  The  difference  between  the  tenacity  of  the  cast 
alloy  and  that  of  the  metal  quenched  at  the  most  desirable 
temperature  is  the  greater  the  less  the  percentage  of 
copper. 

Copper-aluminum  alloys.  —  A  very  careful  investiga- 
tion of  copper-aluminum  alloys  was  made  by  the  Alloys 
Research  Committee  of  the  British  Institution  of  Mechan- 
ical Engineers,  reported  in  the  Proceedings,  1907.  In  the 
summary  of  conclusions  it  is  stated  that  the  limit  of 
industrially  serviceable  alloys  must  be  placed  at  11  per 
cent  of  aluminum. 

Fig.    36   gives   an   equilibrium   diagram  f   of   copper- 

*  See  "  Alloys  "  by  A.  H.  Sexton,  p.  136. 

t  See  The  Constitution  of  the  Aluminum  Bronzes,  by  B.  E. 
Curry,  Journal  of  Physical  Chemistry,  Vol.  XI,  p.  425. 


NON-FERROUS  ALLOYS  187 

aluminum    and    corresponding    strength    and    ductility 
curves  from  the  A.R.C.  report.    The  aluminum  limit  in 
the  diagrams  is  15  per  cent. 
In  the  lower  diagram: 

Full  line  A  gives  strength  of  sand  castings; 

Full  line  B  gives  strength  of  chill  castings; 

Full  line  C  gives  strength  of  rolled  bars; 

Broken  line  A  gives  elongation  in  2  inches  of  sand 

castings; 
Broken    line   B    gives    elongation    in    2    inches    of 

chill  castings; 
Broken  line  C  gives  elongation  in  2  inches  of  rolled 

bars. 

The  very  high  values  of  elongation  may  be  accounted  for 
in  part  by  the  fact  that  the  original  length  of  tested 
portion  was  2  inches.  The  useful  range  may  be  divided 
into  two  parts:  aluminum  0  per  cent  to  8  per  cent,  and 
aluminum  8  to  11  per  cent.  In  the  first  division,  increase 
in  aluminum  content  causes  steady  increase  in  tensile 
strength,  and,  up  to  about  7J  per  cent  aluminum,  steady 
increase  in  ductility.  In  the  second  division,  the  strength 
rises  more  steeply  while  the  ductility  falls  steeply  to  a 
value  corresponding  to  great  brittleness  at  11  per  cent 
aluminum. 

The  first  division  gives  strong,  very  ductile,  shock- 
resistant  alloys. 

The  second  division  gives  more  desirable  alloys  where 
ductility  should  be  sacrificed  to  gain  greater  strength. 

Comparison  of  full  and  broken  curves  A  and  B  shows 
that  chill  casting  does  not  give  any  strength  or  ductility 
advantage  over  sand  casting  up  to  8  per  cent  aluminum, 
but  that  between  8  and  11  per  cent  aluminum  chill  cast- 
ing increases  both  strength  and  ductility.  Curves  C 


188 


MATERIALS  OF    MACHINES 


show  that  rolling  increases  both  strength  and  ductility  in 
the  first  division,  but  that  in  the  second  division  rolling 
gives  about  the  same  results  as  chill  casting. 

Two  kinds  of  heat  treatment  were  applied  to  the  sand 


2012 


1832 


FIG.  36. 


castings,  the  chill  castings  and  the  rolled  bars;  they  were 
heated  to  about  1472°  F.  and  cooled  slowly;  they  were 
heated  to  about  1472°  F.  and  quenched  in  water. 

Neither  slow  cooling  nor  quenching  from  1472°  F.  seems 


NON-FERROUS  ALLOYS  189 

to  produce  very  great  change  in  strength  or  ductility  of 
sand  castings. 

In  case  of  chilled  castings  slow  cooling  from  1472°  F. 
has  little  effect  up  to  8  per  cent  alloys,  but  from  8  per  cent 
aluminum  to  11  per  cent  both  strength  and  ductility  seem 
to  be  somewhat  reduced.  Quenching  from  1472°  F.  seems 
to  have  practically  no  effect  upon  strength  and  ductility. 

Rolled  bars  cooled  slowly  from  1472°  F.  show  increasing 
decrease  in  strength  throughout  the  entire  range,  and 
increase  in  ductility  until  near  the  11  per  cent  limit.  The 
effect  of  quenching  rolled  bars  from  1472°  F.  seems  to  be 
to  decrease  strength  and  increase  ductility  up  to  8  per  cent 
aluminum  and  from  8  to  11  per  cent  aluminum  to  increase 
strength  and  to  decrease  ductility. 

A  comparative  study  of  Figs.  32,  33  and  36  shows  that 
there  is  great  similarity  among  the  copper-zinc,  the  copper- 
tin  and  the  copper-aluminum  alloys.  In  each  case  there 
are  two  useful  ranges;  one  of  medium  strength  and  high 
ductility,  and  hence  of  high  shock  resistance,  another  of 
rapidly  increasing  strength  and  rapidly  decreasing  duc- 
tility. 

The  first  range  in  each  case  corresponds  nearly  to  the 
field  of  a  solution  of  the  equilibrium  diagram,  while  the 
second  range  corresponds  to  the  introduction  of  some 
other  solution  of  the  constituents  which  in  increasing  pro- 
portion rapidly  reduces  the  alloys  to  brittleness  and 
uselessness. 

Kalchoids.  —  Dr.  Thurston  made  a  very  full  series  of 
experiments  on  the  ternary  alloys  of  copper,  tin  and  zinc, 
which  he  called  kalchoids.  He  represented  the  whole  field 
of  possible  combinations  of  the  three  metals  by  an  equi- 
lateral triangular  area.  Many  points  at  equal  distances 
from  each  other  were  located  in  this  area,  and  each  rep- 
resented an  alloy  with  certain  proportions  of  the  three 


190  MATERIALS  OF  MACHINES 

constituents.  Alloys  were  made  corresponding  to  each 
point,  and  tested.  At  each  point  was  erected  a  piece  of 
wire  whose  height  represented  the  strength  of  the  alloy 
represented  by  the  point.  Plastic  material  was  then  filled 
in  between  the  wires,  and  its  surface  was  molded  so  that 
the  points  of  the  wire  just  showed  through.  This  surface 
represented  topographically  the  varying  strength  of  all 
possible  mixtures  of  copper,  tin  and  zinc,  and  the  alloy 
of  maximum  strength  was  thereby  located.  (See  Thurs- 
ton's  "  Textbook  of  the  Materials  of  Construction," 
page  466.) 

Phosphor  bronze.  —  When  any  alloy  containing  a 
high  percentage  of  copper  is  melted  in  contact  with  the 
air,  there  is  a  strong  tendency  to  form  copper  oxide,  the 
affinity  of  copper  for  oxygen  being  exceedingly  strong. 
If  the  cooled  alloy  contains  copper  oxide,  it  is  weak  and 
brittle,  just  as  iron  containing  iron  oxide  is  weak  and 
brittle.  Copper  alloys  are  usually  melted  with  charcoal 
upon  the  surface  to  prevent  oxidation,  but  the  prevention 
is  not  complete.  If  phosphorus  is  added  to  the  alloy 
just  before  pouring,  the  copper  oxide  is  reduced  and  phos- 
phoric acid  is  formed,  i.e.,  the  alloy  is  purified  by  the 
fluxing  action  of  the  phosphorus.  This  increases  both  the 
strength  and  ductility  of  the  alloy.  If  an  excess  of  phos- 
phorus is  added,  part  of  it  may  combine  with  the  alloy 
and  increase  its  strength  and  ductility,  but  the  proportion 
of  phosphorus  should  not  exceed  0.1  per  cent  or  brittle- 
ness  results;  it  is  probable  that  the  chief  value  of  its 
presence  is  to  prevent  the  formation  of  oxide  of  copper 
during  remelting. 

Manganese  bronze  is  made  either  by  fusing  together 
(a)  copper  and  black  oxide  of  manganese,  or  (6)  copper 
or  bronze  and  ferromanganese.  In  the  first  case  the 
product  is  an  alloy  of  copper,  manganese  and  iron,  and 


NON-FERROUS  ALLOYS  191 

in  the  second,  an  alloy  of  copper,  tin,  manganese  and  iron. 
Some  of  the  manganese  is  effective  in  removing,  or  pre- 
venting the  formation  of,  oxide  of  copper,  while  the  re- 
mainder combines  with  the  copper  or  bronze  to  give  it 
very  greatly  increased  strength,  ductility  and  toughness. 
A  manganese  bronze,  copper  83.45  per  cent,  manganese, 
13.48  per  cent,  iron,  1.24  per  cent,  has  a  strength  and 
ductility  equal  to  that  of  open-hearth  steel  with  0.2  per 
cent  carbon.  It  is  much  used  for  marine  propeller- 
wheels  because  it  does  not  corrode  easily. 

All  of  the  useful  copper  alloys  are  more  or  less  forgeable. 
"Muntz  metal,"  copper  60,  zinc  40,  is  rolled  at  a  red  heat 
into  plates  for  sheathing  ships,  and  into  forms  for  bolts 
and  other  fastenings.  It  is  stronger,  cheaper  and  more 
durable  than  pure  copper.  The  working  of  Muntz  metal 
at  red  heat  is  possible  because  it  is  in  the  |8  field*  at  that 
temperature,  and  the  0  solution  is  ductile.  On  cooling, 
however,  it  enters  the  a  +  7  field  losing  ductility.  The 
effect  of  cold  working  upon  the  copper  alloys  is  similar 
to  that  upon  iron  and  steel;  viz.,  the  strength  and  hard- 
ness are  increased  and  the  ductility  is  decreased;  hence 
the  material  is  more  brittle.  This  will  be  clear  on  com- 
paring hard-drawn  brass  wire  with  the  same  wire  after 
annealing. 

Brass  and  bronze  of  different  composition  are  used  for 
journal  boxes,  but  modern  practice  favors  a  box  of  cast 
iron,  brass  or  bronze,  with  a  lining  of  softer  metal,  usually 
a  white  alloy. 

One  group  of  these  white  alloys  is  made  up  of  tin  with 
small  proportions  of  copper  or  antimony  or  both  to  pro- 
duce strength  and  hardness. 

Thus  "  Babbitt  metal  "  consists  of  tin  about  89  per 
cent,  copper  2  to  4  per  cent  and  antimony  about  7.5  to 
*  See  Fig.  32. 


192  MATERIALS  OF  MACHINES 

9  per  cent.     This  is  an  excellent  bearing  alloy,  but  it  is 
expensive. 

Another  group  consists  of  lead  hardened  with  antimony. 
The  antimony  varies  from  10  per  cent  to  20  per  cent  and 
sometimes  a  small  amount  of  tin  is  added.  This  alloy 
is  comparatively  inexpensive. 


CHAPTER  XII 
SELECTION   OF  MATERIALS  FOR  MACHINES 

The  more  important  materials  used  in  machine  con- 
struction may  be  brought  together  as  follows: 

1.  High-speed  steel. 

2.  High-carbon  steel. 

3.  Mild   steel,   produced  by  the  Bessemer  or  open- 
hearth  process. 

4.  Special  structural  steel. 

5.  Wrought  iron. 

6.  Cast  iron. 

7.  Malleableized  cast  iron. 

8.  Steel  castings. 

9.  Brass  or  bronze. 

10.  White  metal.     This  name  includes  all  of  the  white 
alloys  used  for  lining  journal-boxes,  etc. 

1.  High-speed  steel  is  valuable  because  of  its  hardness 
and  toughness  which  fit  it  for  cutting  tools  for  metals, 
and  especially  because  it  retains  these  qualities  when 
raised  to  red  heat.     This  makes  very  high  cutting  speed 
possible  with  corresponding  increase  in  output  of  metal 
cutting  machines.     This  steel  is  very  expensive  because 
of  its  constituents,  its  manufacture  and  its  heat  treat- 
ment, but  the  expense  is  amply  justified  by  the  results. 

2.  High-carbon  steel  is  still  used  for  a  wide  range  of 
cutting  tools  and  it  is  valuable  because  of  its  quality  of 
hardening  and   tempering.     It  is   also   useful  for  stress 
members  where  great  strength  combined  with  medium 

193 


194  MATERIALS  OF  MACHINES 

ductility  is  of  prime  importance;   also  for  springs  because 
of  its  wide  natural  or  artificial  elastic  range. 

3.  Mild  steel  by  reason  of  its  medium  strength,  high 
ductility  and  low  price  is  used  in  structures  and  machines 
for  all  except  special  service. 

4.  Special  structural  steels  containing  nickel,  chromium, 
vanadium,  etc.,  are  useful  because  their  high  strength,  duc- 
tility and  shock  resistance  fit  them  for  light  shock-en- 
during structures  like  motor  cars  and  aeroplanes,  as  well 
as  for  great  shock  resistance  in  projectiles  and  armor 
plates. 

5.  Wrought  iron  has  the  advantage  over  mild  steel 
that  it  forges  much  more  easily,  probably  because  of  its 
slag  content,  and  hence  it  is  used  quite  extensively  for 
hand-forging.     It  is  also  claimed  that  it  is  less  subject  to 
destruction  by  corrosion  and  hence  it  still  competes  with 
steel  for  pipes,  boiler  tubes  and  similar  service. 

6.  Cast  iron  is  almost  universally  used  for  forms  that 
must  be  shaped  by  casting,  especially  where  great  weight  is 
unobjectionable  or  where  great  weight  is  desirable,  as  in 
fly-wheels,  machine  beds  or  frames.     When  cast  forms 
require  .both  strength  and  lightness,  cast  iron  gives  place 
to  other  material. 

7.  Malleable  iron  is  used  for  cast  forms  that  require 
great  shock-resistance  —  for  purposes  for  which  brittle 
cast  iron  is  unsafe. 

8.  Steel   castings  meet  the   same  need   as  malleable 
cast  iron,  but  the  process  for  production  and  the  nature 
of  the  material   adapts  them  to  much  larger  machine 
members. 

9.  The  copper  alloys  have  no  advantage  over  mild 
steel   in    strength,    ductility  and    shock  resistance,    and 
hence,  since  the  cost  is  very  much  greater  there  must  be 
some  other  advantage  to  lead  to  their  selection  for  machine 


SELECTION  OF  MATERIALS  195 

parts.  These  alloys  are  much  safer  against  oxidation 
than  steel  and  hence  are  used  for  condenser  tubes,  often 
for  feed  pipes,  for  valves  of  many  kinds  and  for  many 
parts  of  mechanisms  subject  to  corrosion.  Moreover  these 
alloys  have  good  anti-friction  qualities  and  are  used  for 
wearing  surfaces. 

10.  The  white  alloys  are  chiefly  useful  for  wearing 
surfaces.  The  surfaces  of  machine  parts  that  move  over 
each  other  under  pressure  are  normally  separated  by  a 
film  of  lubricating  material.  But  under  exceptional  con- 
ditions the  metallic  surfaces  themselves  may  come  into 
contact;  when  this  occurs  the  danger  of  roughening  or 
destroying  the  surfaces  depends  somewhat  upon  the  excel- 
lence of  the  surface  and  kind  of  material. 

A  material  may  be  well  adapted  for  wearing  surfaces 
because  of  (a)  hardness,  (6)  slipperiness,  (c)  homogeneous- 
ness  or  (d)  because  it  is  partly  composed  of  lubricating 
material. 

Thus,  (a)  hardened  tool-steel  is  difficult  to  roughen 
because  of  its  hardness;  (6)  white  metal,  though  soft,  is 
difficult  to  roughen,  because  the  roughening  agent  slides 
over  the  slippery  surface;  (c)  mild  steel  would  have  less 
tendency  to  roughen  an  engaging  surface  than  wrought 
iron,  because  the  former  has  a  homogeneous  surface, 
while  the  latter  carries  streaks  of  gritty  cinder;  (d)  cast 
iron  tends  to  wear  smooth  rather  than  rough,  because  it 
contains  graphitic  carbon,  a  lubricating  material. 

The  ideal  for  rotating  surfaces  would  be  a  hardened, 
accurately  ground,  crucible-steel  journal,  with  its  bearing 
lined  with  high-grade  white  metal.  But  here  the  question 
of  cost  enters,  for  the  cost  of  the  journal  specified  includes 
high  first  cost  for  the  crucible  steel,  the  cost  for  hard- 
ening, and  a  cost  incident  upon  the  loss  of  expensive 
parts  through  cracking  in  the  process  of  hardening.  In 


196  MATERIALS  OF  MACHINES 

addition  to  this,  an  expensive  plant  is  required  for  the 
hardening  of  large  journals. 

In  standard  practice  mild  steel  journals  are  used  with 
bearings  lined  with  white  metal;  but  there  are  often  con- 
ditions that  lead  to  the  use  of  other  materials. 

Sliding  surfaces  in  machines  are  often  formed  upon 
cast-iron  members,  and  the  engaging  surface  is  also  of 
cast  iron.  The  frictional  loss  may  be  reduced  by  giving 
one  surface  a  white-metal  covering. 

To  illustrate  the  selection  of  materials  for  machine 
parts,  a  few  typical  examples  will  be  discussed. 

The  cylinder  of  a  steam-engine,  with  its  ports  and 
its  connected  steam-chest,  is  of  such  complicated  form 
that  it  is  almost  impossible  to  shape  it  by  forging;  or 
if  the  forging  were  possible,  it  would  be  too  expensive. 
The  possible  materials  which  may  be  used  for  such  a 
cylinder  are,  therefore,  only  those  which  are  shaped  by 
casting.  Brass  and  bronze  would  have  no  advantage 
over  cast  iron,  and  would  cost  about  ten  times  as  much. 
They  are,  therefore,  out  of  the  question.  Steel  casting 
might  be  used,  but  the  first  cost  of  the  material  would  be 
somewhat  greater,  and  the  cost  of  working  in  the  machine- 
shop  would  be  very  much  greater.  Additional  strength 
and  resilience  would  be  gained,  but  this  is  unnecessary,  as 
cylinders,  even  for  very  high  pressures,  can  be  made  of 
cast  iron,  amply  strong  and  resilient,  and  yet  not  objec- 
tionably thick.  Moreover,  cast  iron  is  one  of  the  very 
best  possible  materials  for  the  wearing  surfaces  of  the 
cylinder  and  valve-seat.  Cylinders  subjected  to  exces- 
sively high  pressure,  as  300  to  700  pounds  per  square 
inch,  should  perhaps  be  made  of  steel  castings,  as,  for 
instance,  the  cylinders  of  pumps  for  pipe-lines,  or  for 
supplying  hydraulic  machinery. 

The  piston-rod  of  a  steam-engine  is  of  mild  steel. 


SELECTION  OF  MATERIALS  197 

The  entire  force  of  the  steam  acting  on  the  piston  must 
be  transmitted  to  the  cross-head  through  the  piston-rod; 
also,  since  the  effective  area  of  the  piston  on  the  crank 
side  equals  the  total  area  of  the  piston  less  the  area  of 
the  rod,  and  since  the  effective  area  needs  to  be  as  large 
as  possible,  the  rod  should  be  as  small  as  possible.  There 
is  always  the  liability  to  shocks,  and,  therefore,  since  the 
rod  must  be  small  and  at  the  same  time  strong,  and  must 
also  be  capable  of  resisting  shocks,  a  material  of  high  unit 
strength  and  of  high  resilience  is  required.  Soft  steel 
is  the  material  which  combines  these  qualities. 

A  steam-engine  cross-head  pin  is  always  made  much 
larger  than  is  necessary  to  safely  resist  shearing,  or  spring- 
ing by  flexure,  to  insure  the  maintenance  of  lubrication; 
cast  iron  might  serve,  then,  as  far  as  strength  and  stiffness 
are  concerned,  and  in  fact  is  sometimes  used.  But  there 
is  another  important  consideration:  because  of  the  vibra- 
tory motion  of  the  connecting-rod  on  the  pin,  there  is  a 
tendency  to  wear  the  pin  oval,  and  when  the  boxes  are 
"keyed  up,"  they  will  bind  when  the  rod  is  in  its  position 
of  greatest  angularity,  if  it  is  properly  adjusted  when  the 
rod  is  on  the  center  line  of  the  engine.  Because  of  this  it 
is  desirable  to  reduce  the  wear  to  a  minimum,  and  this 
points  to  the  selection  of  a  hard  material.  Hardened  tool- 
steel  might  be  used,  but  it  is  more  expensive  than  soft 
steel  or  wrought  iron,  and  there  is  the  danger  of  hidden 
cracks,  resulting  from  the  hardening,  which  may  cause 
accident.  If  soft  steel  is  case-hardened,  it  will  combine 
a  hard  surface  to  resist  wear  with  a  soft  resilient  core, 
free  from  the  danger  of  cracks.  Wrought  iron  case- 
hardened  might  be  used,  but  wrought  iron,  because  of  the 
method  of  manufacture,  has  streaks  of  cinder  in  its  sur- 
face, and  lacks  the  homogeneity  of  the  steel,  and  is  there- 
fore harder  to  make,  and  to  keep  truly  cylindrical.  It 


198  MATERIALS  OF  MACHINES 

therefore  should  not  be  used  where  perfection  of  bearing 
and  accuracy  of  movement  are  essential. 

The  connecting-rod  of  a  steam-engine  is  subjected  to 
the  alternate  tension  and  compression  resulting  from  the 
pressure  on  the  piston,  and  also  to  a  flexure  stress  due  to 
its  vibratory  motion.  These  stresses  are  very  severe, 
and  there  is  also  liability  to  shock.  The  material  of  the 
rod  should  be  strong  and  resilient,  and  soft  steel  would 
naturally  be  selected,  since  it  is  a  forgeable  material. 
But  there  is  another  important  consideration;  the  rod  is 
to  be  finished,  and  wrought  iron  is  much  more  cheaply 
worked  in  the  machine-shop  than  soft  steel,  and  the 
expense  of  forging  is  also  much  less.  The  lack  of  homo- 
geneity is  of  no  importance,  as  no  part  of  the  rod  is  a 
bearing-surface.  Many  connecting-rods  are  made  of 
steel  casting,  and  finished  by  painting.  This  makes  a 
cheaper  rod,  but  there  is  always  the  danger  of  hidden 
defects,  like  cracks,  due  to  the  excessive  shrinkage,  or 
blow  holes,  which  may  weaken  the  rod  enough  to  cause 
accident. 

The  cross-head  of  a  steam-engine  is  composed  of 
two  parts :  (a)  that  which  serves  to  transmit  the  pressure 
from  the  piston-rod  to  the  cross-head  pin,  and  (6)  that 
which  engages  with  the  guide  to  produce  rectilinear  mo- 
tion. The  stresses  on  (a)  are  severe,  and  there  is  lia- 
bility to  severe  shock;  hence  it  must  be  of  strong  resilient 
material;  the  stresses  on  (6),  however,  are  less,  but  it 
must  be  of  material  which  will  run  well  with  the  guide, 
which  is  usually  of  cast  iron,  being  a  part  of  the  engine- 
bed.  The  cross-head  may  be  made  of  materials  as  follows: 
(a)  may  be  made  of  forged  wrought  iron  or  soft  steel, 
and  (6)  may  be  of  cast  iron  bolted  to  (a),  or  the  whole 
cross-head  may  be  made  of  cast  iron,  the  part  (a)  being 
made  enough  larger  than  before  to  be  sufficiently  strong; 


SELECTION  OF  MATERIALS  199 

or  the  cross-head  may  be  made  a  casting  of  steel  and  a 
"shoe  "  or  "gib  "  of  cast  iron  or  brass  may  be  added  to 
provide  a  proper  surface  to  run  in  contact  with  the  guide. 

The  crank-pin  of  a  steam-engine  is  subjected  to  the 
same  stress  as  the  cross-head  pin,  and  the  velocity  of 
rubbing  surface  is  very  much  greater,  hence  the  ten- 
dency to  wear  is  greater.  But  the  tendency  to  wear  "out 
of  round  "  is  less  and  therefore  there  is  less  interference 
with  the  correct  adjustment  of  the  boxes;  hence  there  is 
less  reason  for  keeping  the  wear  a  minimum;  a  good 
journal  surface  is  necessary,  and  soft  steel  is  used  without 
case-hardening. 

The  main  shaft  of  a  steam-engine  needs  to  be  strong 
and  rigid  to  resist  a  combination  of  severe  stresses,  i.e., 
the  torsional  and  transverse  stress  from  the  connecting- 
rod,  and  the  transverse  stress  due  to  the  weight  of  the 
fly-wheel,  and  the  belt  tension.  It  must  also  afford  a 
good  journal  surface,  and  for  these  reasons  it  is  made  of 
soft  steel. 

The  function  of  the  fly-wheel  of  a  steam-engine  is  to 
adapt  a  varying  effort  to  a  constant  resistance,  and  it 
does  this  by  absorbing  and  giving  out  energy  periodically 
by  virtue  of  its  inertia,  which  is  proportional  to  its  weight; 
it  therefore  needs,  above  all  things,  to  be  heavy;  it  also 
needs  to  be  able  to  resist  the  bursting  tendency  of  the 
centrifugal  force  due  to  its  rotation.  The  most  suitable 
material  therefore  is  that  which  gives  the  greatest  weight 
in  the  required  form,  with  the  required  strength,  for  the 
least  money;  and  cast  iron  best  fulfills  these  require- 
ments. 

An  engine  bed  or  frame,  when  it  is  in  one  piece,  is  of 
cast  iron,  and  the  reasons  are  obvious:  its  form  is  com- 
plex, and  could  only  be  produced  by  casting;  weight  is 
not  objectionable,  but  rather  an  advantage,  since  it 


200  MATERIALS  OF  MACHINES 

absorbs  vibrations;  cast  iron  is  amply  strong,  and  affords 
good  wearing  surfaces  for  the  cross-head  guides.  Wrought 
iron  is  used  for  engine-beds,  where  vibrations  are  less 
important,  as  in  the  locomotive,  and  where  lightness  and 
compactness  are  very  desirable,  as  in  some  marine  en- 
gines. The  beds  of  some  large  roll-train  and  blowing 
engines  are  built  up  of  wrought  and  cast  iron. 

The  journal-bearings,  or  boxes  for  the  cross-head  pin, 
the  crank-pin  and  the  journals  of  the  main  shaft  are 
now  usually  made  of  cast  iron  or  brass,  with  a  babbitt- 
metal  lining,  because  good  babbitt  metal  (tin  80,  copper 
10,  antimony  10)  is  found  to  be  a  better  bearing  metal 
than  brass,  i.e.,  it  runs  with  less  tendency  to  heat;  and 
in  the  case  of  the  cutting  out  of  the  surface,  the  babbitt- 
lined  box  is  far  more  quickly  and  cheaply  renewed  than 
the  solid  brass  box. 

The  eccentric  and  its  strap  are  almost  invariably 
made  of  cast  iron,  because  they  are  forms  which  are 
forged  with  difficulty,  and  the  cast  iron  affords  ample 
strength  and  excellent  wearing  surfaces.  The  eccen- 
tric-rod, on  the  other  hand,  would  be  cumbersome  and 
ugly  in  appearance  if  it  were  made  of  cast  iron  and  given 
sufficient  strength.  It  is  a  form  which  may  be  easily 
either  forged  or  cast,  and  is  made  of  forged  wrought  iron 
or  steel,  or  of  cast  steel,  or  of  malleableized  cast  iron. 
Rocker-arms,  also,  when  they  are  used,  require  to  be  of  a 
resilient  material,  and  when  of  simple  form  may  be  forged 
of  wrought  iron  or  steel,  and  when  of  more  complex  form 
may  be  of  malleableized  cast  iron,  or  steel  casting.  The 
valve  is  usually  of  somewhat  complex  form,  and  needs  to 
wear  well  with  the  cast-iron  valve-seat,  and  is  almost 
invariably  of  cast  iron. 

Considerations  similar  to  those  above  apply  to  the 
selection  of  proper  material  for  the  parts  of  machine 


SELECTION  OF  MATERIALS  201 

tools.  Thus,  in  the  case  of  a  lathe,  the  bed,  legs,  head 
and  tail-stock,  cone,  gears,  etc.,  are  of  cast  iron,  because 
they  are  all  forms  which  are  most  cheaply  and  satisfac- 
torily produced  by  casting,  and  the  cast  iron  affords  the 
required  strength  and  stiffness,  and  satisfactory  wearing 
surface,  where  they  are  required.  Such  parts  as  lead- 
screws,  feed-rods  and  other  parts  which  are  subjected  to 
considerable  stress,  and  have  great  length  relatively  to 
their  lateral  dimensions,  are  made  necessarily  of  wrought 
iron  or  steel.  Many  of  these  parts  may  be  finished  in  the 
machine-shop  directly  from  merchant-bar  stock,  thus 
saving  expense  for  forging. 

The  material  for  the  parts  of  planing,  milling,  and 
drilling  machines  are  determined  from  exactly  similar 
considerations. 

Spindles,  however,  require  special  attention.  In 
lathes,  milling  and  grinding  machines  the  accuracy  of 
the  work  produced  depends  largely  upon  the  accuracy 
of  the  spindle. 

The  vital  point  therefore  is  to  maintain  this  accuracy, 
i.e.,  to  prevent  wear  as  far  as  possible.  It  would  seem 
then  that  hardened  tool-steel  would  be  the  best  material. 
But  since  only  a  very  small  amount  of  stock  can  be  re- 
moved by  the  grinding  machine  after  the  piece  is  hard- 
ened, the  spindle  must  be  roughed  out  very  nearly  to  size 
before  it  is  hardened;  this  involves  a  very  considerable 
expense,  and  there  is  danger  that  it  may  crack  in  harden- 
ing, or  spring  so  as  not  to  hold  up  to  finish,  in  which  case 
the  loss  is  great,  and  it  is  found  that  the  risk  cannot  be 
taken.  The  next  best  thing  is  to  specify  machinery  steel 
high  in  carbon  (say  0.4  per  cent),  and  to  use  this  harder 
material  for  the  spindle  without  hardening.  In  milling- 
machines  and  in  some  lathes  the  main  spindle-box  is  solid, 
of  tool-steel,  hardened  and  ground  (the  risk  of  loss  being 


202  MATERIALS  OF  MACHINES 

less  in  this  case),  and  the  spindle  as  before  is  of  0.4  per  cent 
carbon  machinery  steel.  The  wear  is  thus  greatly  re- 
duced, and  the  possibility  of  wear  after  long  use  is  provided 
against  by  making  the  bearing  taper,  and  providing  end 
adjustment.  The  spindles  of  very  large  lathes  are  some- 
times made  of  cast  iron,  because  forged  material  would  be 
too  expensive.  The  wear  is  reduced  by  making  the  jour- 
nals very  large. 

In  the  steam  or  hydraulic  riveter  the  main  frame 
which  supports  the  cylinder,  and  carries  the  guide  for  the 
moving  die,  may  be  of  any  reasonable  size,  and  therefore 
can  be  made  strong  enough  to  resist  even  the  very  great 
forces  applied  to  it,  if  the  material  used  is  cast  iron. 
But  the  " stake,"  the  member  which  carries  the  stationary 
die,  must  resist  exactly  the  same  forces  as  the  main  frame, 
and  must  also  be  small  enough  so  that  small  boiler-shells, 
and  even  flues,  can  be  lowered  over  it  to  be  riveted.  The 
"  stake  "  is  therefore  of  forged  wrought  iron  or  steel, 
or  else  a  steel  casting. 

Suppose  that  in  a  machine  there  is  need  of  a  gear  and 
pinion  whose  velocity  ratio  is  8  to  1,  and  that  the  force 
transmitted  is  large.  A  tooth  of  the  pinion  comes  into 
action  eight  times  as  often  as  a  tooth  of  the  gear,  and 
therefore  would  wear  out  in  one-eighth  of  the  time  if 
both  were  of  the  same  material;  then,  too,  the  form  of  the 
pinion-tooth  in  most  systems  of  gearing  is  such  that  it  is 
much  weaker  than  the  gear-tooth.  The  material  for  the 
pinion  needs,  therefore,  not  only  to  be  stronger,  but  also 
better  able  to  resist  wear.  The  gear  is  made  of  cast  iron; 
if  the  teeth  are  cut,  the  pinion  may  be  made  of  forged 
steel;  if  the  teeth  are  cast  and  used  without  "  tooling," 
the  pinion  may  be  made  a  steel  casting. 

Material  for  Springs.  —  Springs  are  useful  as  machine 
parts  because  of  their  capacity  for  yielding  without  taking 


SELECTION  OF  MATERIALS  203 

permanent  set.  The  yielding,  therefore,  must  occur  with 
stresses  that  do  not  exceed  the  elastic  limit.  Clearly, 
then,  the  material  with  large  elastic  range,  i.e.,  with  high 
elastic  limit,  is  the  best  material  for  spring  machine- 
members. 

Crucible-steel  has  the  highest  normal  elastic  limit,  and 
this  limit  is  raised  by  hardening  and  tempering.  This  is 
the  most  commonly  used  material.  Untreated  mild  steel 
may  also  be  used,  but  with  given  stress  the  spring  must 
have  greater  weight  than  if  higher  carbon  steel  were  used. 
The  steel  may  have  its  normal  elastic  limit  artificially 
raised  by  cold  working  (cold  rolling  or  wire-drawing), 
and  this  improves  it  as  a  spring  material.  Brass,  bronze 
and  other  alloys  are  used  for  springs,  but  usually  in  the 
form  of  hard-drawn  wire  with  an  artificial  elastic  limit. 


INDEX 


PAGE 

Acid,  neutral  or  basic  lining  for  furnaces 30 

Alcohol  as  a  solution  of  the  future  fuel  problem 19 

Alcohol  not  an  important  industrial  fuel 18 

Aluminum 12,  88 

Aluminum  and  copper,  equilibrium  diagram  of 188 

Aluminum  as  a  fuel 13 

Aluminum  in  brass 181 

Annealing 69,  158,  167 

Annealing  brass 179 

Annealing  forgings 172 

Annealing  restores  strength  and  elasticity  in  steel 161 

Annealing  steel  castings 172 

Anthracite  coal,  —  chemical  changes  while  in  process  of  for- 
mation   14 

Antimony  and  arsenic  in  brass 182 

Arc  furnace 26 

Arsenic  and  antimony  in  brass 182 

Arsenic  and  copper,  presence  of,  in  steel 151 

Artificial  fuels 2,  15 

Artificial  gas,  processes  for  production  of 20 

Artificial  modulus  of  elasticity 103 

Austenite 166 

Babbitt  metal 191 

Basic,  acid  or  neutral  lining  for  furnaces  . 30 

Basic  Bessemer  process 61 

Basic  Bessemer  process,  graphical  representation  of 65 

Basic  Bessemer  process,  silicon  an  undesirable  element  in ....  62 

Bauschinger's  experiments  on  repeated  stresses 159 

Bauxite 34 

Belgian  process  for  smelting  zinc 87 

Bessemer  converter,  lining  of 61 

205 


206  INDEX 

PAGE 

Bessemer  process 12,  58 

Bituminous  coal,  composition  of 15 

Blast-furnace,  chemical  charges  in 42 

Blast-furnace,  function  of 40 

Blast-furnace  stack,  continuous  flow  of  gas  from 45 

Blast-furnaces,  charcoal  as  fuel  for 48 

Blister  copper 80 

Brass 177 

Brass,  annealing 179 

Brass,  antimony  and  arsenic  in 182 

Brass,  effect  of  cold  working  on 180 

Brass,  iron  in 181 

Brass,  manganese  in 181 

Brass,  oxygen  in 182 

British  thermal  unit 2 

Brittle  material 109 

Brittleness  and  porosity,  how  to  avoid '. 74 

Bronzes,  Guillet's  conclusions  on  heat  treatment  of 186 

Bronzes,  heat  treatment  for 184 

Bronzes  or  copper-tin  alloys 182 

"  Burnt "  scrap  iron 142 

Calamine 86 

Calcining  ore 38 

Calorific  power  of  carbon  monoxide 3 

Calorific  power  of  combustible 2,  3 

Carbon 34 

Carbon  and  iron,  equilibrium  diagram  of 110,  112 

Carbon  between  the  states  of  cementite  and  graphite,  im- 
portance of  control  of 123 

Carbon,  chief  factor  controlling  physical  properties  of  steel ...  149 

Carbon,  combined,  in  cast  iron 121 

Carbon,  graphite  an  allotropic  form  of 122 

Carbon,  graphite  in  cast  iron 121 

Carbon,  introduction  of,  into  the  iron  sponge 43 

Carbon,  limit  of,  in  solid  solution  in  iron 115 

Carbon  monoxide,  calorific  power  of 3 

Carbon  monoxide  gas,  temperature  produced  by  burning ....  6 

Carbon,  removal  of,  in  puddling  process 55 

Case-hardening 172 

Cast  iron  and  steel,  chemical  difference  between 144 


INDEX  207 

PAGE 

Cast  iron  and  steel,  temperatures  of  solidification  of 75 

Cast  iron,  chilling 125 

Cast  iron,  combined  carbon  in 121 

Cast  iron,  composition  of 72,  121 

Cast  iron,  densities  of  different  grades  of 139 

Cast  iron,  graphite  carbon  in 121 

Cast  iron,  introduction  of  sulphur  into 43 

Cast  iron,  malleable,  composition  of 133 

Cast  iron,  manganese  in 127 

Cast  iron,  phosphorus  in 131 

Cast  iron,  rate  of  cooling "  125 

Cast  iron,  silicon  in 128 

Cast  iron,  stress-elongation  diagram 101 

Cast  iron,  structural  steel  and  tool-steel,  chemical  comparison 

of 144 

Cast  iron,  sulphur  in 127 

Cast  iron,  two  forms  of  carbon  in 121 

Cast  iron,  white 122 

Cellulose  and  starch 18 

Cementation  process  for  making  tool-steel 57 

Cementite,  influence  of,  on  cast  iron 121 

Charcoal 15 

Charcoal  as  a  fuel  for  blast-furnaces 48 

Chemical  changes  in  the  blast-furnace 42 

Chemical  comparison  of  structural  steel,  tool-steel  and  cast 

iron 144 

Chemical  difference  between  steel  and  cast  iron 144 

Chilling  cast  iron 125 

Chrome  steel 153 

Chromite 33 

Chromium 152 

Coke 15 

Coke,  descent  of,  in  blast-furnace 44 

Cold  rolling,  effect  of,  on  iron,  Thurston's  experiments 158 

Cold  working  of  steel,  effect  of 156 

Combination  inductive  and  resistance  furnace 29 

Combined  carbon  in  cast  iron 121 

Combustible,  calorific  power  of 2,  3 

Combustion,  complete 2 

Combustion,  heat  changes  during 5 

Combustion  products,  nature  of 3 


208  INDEX 

PAGE 

Combustion,  temperatures  resulting  from '. 3 

Compression 107 

Compressive  stress 92 

Connecting-rod  of  steam-engine,  material  for 198 

Converter,  Bessemer 58 

Cooling  cast  iron,  rate  of 125 

Copper-aluminum  alloys 186 

Copper  and  aluminum,  equilibrium  diagram  of 188 

Copper  and  tin,  equilibrium  diagram  of 183 

Copper  and  zinc,  equilibrium  diagram  of 178 

Copper,  oxides,  carbonates  and  silicates  of 78 

Copper,  sources  of 77 

Copper  sulphide  ores 79 

Copper-tin  alloy,  best  composition  for  strength  and  ductility  183 
Copper-tin  alloys,  Shepherd-Upton  tests  of  physical  qualities 

of 182 

Copper-zinc  alloy,  tensile  strength  and  ductility  of 178 

Copper-zinc  alloys 177 

Copper-zinc,  copper-tin  and  copper-aluminum  alloys,  similarity 

of 189 

Crank-pin  of  steam-engine,  material  for 199 

Cross-head  pin  for  steam-engine,  materials  for 197 

Cross-head  of  steam-engine,  material  for 198 

Crucible  process  for  making  tool-steel 57 

Crucible  steel,  elastic  limits  of 203 

Crude  petroleum 17 

Cupola  furnace,  melting  pig  iron  in 50 

Cylinder  of  steam-engine,  materials  for 196 

Deformation  and  stress,  simultaneous  values  of 93 

Deformation  proportional  to  stress  within  elastic  limit 94 

Delta  metal 182 

Diagram  of  cast-iron  stress-elongation 101 

Diagram  of  Diederichs'  blast-furnace 46 

Diagram  of  mild  steel  stress-deformation 98 

Diederichs'  blast-furnace  diagram 46 

Diffusion 114 

Dilution,  reduction  by 68 

Dry  blast 48 

Dry-air  blast,  Gay  ley's  plant  for  furnishing 49 

Dry  puddling 54 


INDEX  209 

PAGE 

Ductile  castings 69 

Ductile  material 109 

Ductility 100 

Ductility  of  steel  castings 76 

Duplex  process 68 

Eccentric,  material  for 200 

Eccentric-rod,  material  for 200 

Eccentric-strap,  material  for 200 

Elastic  deformation 95,  99 

Elastic  limit 94 

Elastic  limit,  artificial 102 

Elastic  range 102 

Elastic  resilience 99 

Elasticity 100 

Elasticity,  definition  of 94 

Electric  furnaces 26 

Engineering  and  metallurgical  processes,  temperatures  in ....  10 

Equilibrium  diagram  of  copper  and  aluminum 188 

Equilibrium  diagram  of  copper  and  tin 183 

Equilibrium  diagram  of  copper  and  zinc 178 

Equilibrium  diagram  of  iron  and  carbon 110,  112 

Experiments  on  hardening  effect  of  remelting  iron 141 

Factors  of  safety  of  brittle  materials 164 

Factors  of  safety  of  steel 163 

Ferrochrome 152 

Ferrosilicon,  use  of,  in  the  foundry  cupola 129 

Ferrovanadium 153 

Fire-clay 32 

Flaws,  repeated  stress  increases  size  of 162 

Fly-wheel  of  steam-engine,  material  for 199 

Forged  material,  internal  stresses  in 154 

Foundry  cupola,  use  of  ferrosilicon  in 129 

Frame  of  steam-engine,  material  for 199 

Franklinite 86 

Fuel,  economy  of,  due  to  hot  blast 47 

Fuels,  preliminary  consideration  of 1 

Galena  ore 81 

Galena,  roasting 81 

Galena,  smelting 82 


210  INDEX 

PAGE 

Galena,  softening 83 

Galena,  treatment  of 81 

Gas  fuel,  advantages  over  solid  fuels 19 

Gayley's  plant  for  furnishing  dry-air  blast 49 

Gear  and  pinion,  material  for 202 

Graphite  an  allotropic  form  of  carbon 122 

Graphite  carbon  in  cast  iron 121 

Guillet's  conclusions  on  heat  treatment  of  bronzes 186 

Hall's  patent  for  metallic  aluminum 88 

Hardening  steel 167,  169 

Harveyized  steel 152 

Heat  available  to  raise  temperature  of  products  of  combustion  7 

Heat  treatment  for  bronzes 184 

Heat  treatment  for  tool-steel 57 

Heat  treatment  of  bronzes,  Guillet's  conclusions  on 186 

Heat  treatment  of  high-speed  steels,  description  of 175 

Heat  treatment  of  steel 165 

High-carbon  steel 152 

High-speed  tool-steel 173 

Homogeneousness  of  materials,  effect  of  lack  of  on  stress- 
deformation  diagram 155 

Hydrogen,  reasons  for  producing  lower  temperature  than  other 

fuels 9 

Hydrogen,  temperature  produced  by  combustion  of 7 

Illuminating-gas  process 20 

Induction  electric  furnace 28 

Iron,  allotropic  forms  of 110 

Iron  and  carbon,  equilibrium  diagram  of 110,  112 

Iron,  crystal  structure  of Ill 

Iron,  early  methods  of  production 39 

Iron,  effect  of  remelting 141 

Iron  in  brass 181 

Iron,  influence  of  carbon  upon 112 

Iron,  limit  of  carbon  in  solid  solution  in 115 

Iron,  liquid 112 

Iron  ores,  composition  of 39 

Iron,  sources  of 38 

Iron  sponge 42 

Iron,  two  forms  of  shrinkage  of 135 


INDEX  211 

PAGE 

Journal-bearings,  material  for 200 

Kalchoids 189 

Kaolin 32 

Kelvin's  experiments  on  repeated  stresses 161 

Lake  Superior  copper 77 

Lead 81 

Lirne 35 

Lime  acting  as  a  flux 42 

Lining  of  Bessemer  converter 61 

Linings  for  furnaces 30 

Liquid  fuels 17 

Low  carbon  steel 152 

Machine  parts,  materials  for 193 

Machine  stress-members,  causes  of  failure  of 163 

Machine  tool  parts,  material  for 200 

Magnesia 35 

Manganese  in  brass 181 

Manganese  in  cast  iron 127 

Manganese,  introduction  of,  into  iron 43 

Main-shaft  of  steam-engine,  material  for 199 

Malleable  castings 69 

Malleable  castings,  analysis  of 72 

Malleable  castings,  tests  of 71 

Malleable  cast  iron,  composition  of 133 

Manganese  bronze 190 

Manganese,  function  of,  in  steel 150 

Martensite 166 

Maximum  stress 95 

Metal-cutting  tools,  four  eras  in  development  of 175 

Metallic  aluminum 88 

Metallic  aluminum,  Hall's  patent  for 88 

Metallurgy  of  copper,  lead,  tin,  zinc  and  aluminum,  outline  of  77 

Metallurgy  of  iron  and  steel 37 

Mild  steel,  stress-deformation  diagram 98 

Modulus  of  elasticity 102 

Modulus  of  elasticity,  artificial 103 

Molds  for  steel  castings 75 

Muntz  metal 180,  191 

Mushet's  self -hardening  steel 174 


212  INDEX 

PAGE 

Native  copper 77 

Nickel,  introduction  of,  into  steel 152 

Neutral,  basic  or  acid  lining  for  furnaces 30 

Neutral  flame 53 

Non-ferrous  alloys 177 

Open-hearth  furnaces 73 

Open-hearth  processes 66 

Oxidizing  flame 53 

Oxygen  in  brass 182 

Pattinson  process  for  lead 84 

Permanent  distortion  and  breakage 91 

Phosphor  bronze 190 

Phosphorus,  effect  of,  on  cooling  steel 151 

Phosphorus  in  cast  iron 131 

Phosphorus,  introduction  of,  into  cast  iron 44 

Phosphorus,  removal  of,  in  puddling  process 55 

Physical  properties  of  steel,  carbon  chief  factor  controlling. ...  149 

Pig  iron,  composition  of 40 

Pig  iron,  composition  of,  for  basic  Bessemer  process 63 

Pig  iron,  grayer,  from  blast-furnace  using  hot  blast 47 

Pig  iron,  melting,  in  cupola  furnace 50 

Pig  iron,  uses  of 50 

Piston-rod  of  steam-engine,  material  for 196 

Planing,  milling  and  drilling  machines,  material  for.  ........  201 

Plant  tissue 13 

Plastic  dolomite  for  daubing  and  patching 36 

Porosity  and  brittleness,  how  to  avoid 74 

Processes  for  making  tool-steel  from  wrought-iron 56 

Products  of  combustion,  heat  available  to  raise  temperature  of.  7 

Producer-gas  process 22 

Puddling  process 51 

Puddling  process,  removal  of  carbon  in 55 

Puddling,  reverberatory  furnace  for 53 

Pulverized  coal 16 

Raw  fuels 13 

Reducing  flame 53 

Refining  process 54 

Refining  steel 167,  168 


INDEX  213 

PAGE 

Refractory  materials 30 

Regenerative  furnace,  Siemen's 66 

Repeated  stress,  effect  of 158 

Repeated  stress,  range  of 160 

Resilience 104 

Resistance  electric  furnace 27 

Reverberatory  furnace  for  puddling 53 

Riveter  frame,  material  for 202 

Roasting  ore 38 

Rocker-arms  of  engine,  material  for 200 

Semi-steel 132 

Shearing  stress 93 

Shepherd-Upton  tests  of  physical  qualities  of  copper-tin  alloys. .  182 

Shrinkage  of  iron,  two  forms  of 135 

Shrinkage  of  iron,  West's  experiment 136 

Siemens-Martin  process 67 

Siemen's  process 67 

Siemen's  regenerative  furnace 66 

Silica 33 

Silicon 12 

Silicon  an  undesirable  element  in  the  basic  Bessemer  process . .  62 

Silicon  in  cast  iron 128 

Silicon,  introduction  of,  into  cast  iron 43 

Solid  fuels 13 

Sorbite 166 

Spiegeleisen 61,  150 

Spindles,  material  for 201 

Spring  tempering 167,  171 

Springs,  material  for 202 

Squeezer  for  removing  slag 55 

Starch  and  cellulose 18 

Steel  and  cast  iron,  chemical  difference  between 144 

Steel  and  cast  iron,  temperatures  of  solidification  of 75 

Steel  castings 73 

Steel  castings,  ductility  of 76 

Steel  castings,  molds  for 75 

Steel  castings,  shock  resistance  of 76 

Steel  castings,  strength  of , 76 

Steel,  effect  of  cold  working  of 156 

Steel,  effect  of  temperature  on 163 


214  INDEX 

PAGE 

Steel  for  castings 75 

Steel,  hot  working  of 171 

Steel  ingots,  reheating  and  working 74 

Steel,  introduction  of  nickel  into 152 

Steel  structure  changes  with  increasing  temperature 165 

Stiffness  of  material 102,  106 

Stoughtoh  converter 73 

Strength  at  elastic  limit 99 

Strength  of  castings,  effect  of  internal  strength  on 139 

Stress  and  deformation,  simultaneous  values  of 93 

Stress  and  deformation,  where  proportionality  of,  ceases 97 

Stress-deformation  diagram 97 

Stress  within  elastic  limit,  deformation  proportional  to 94 

Structural  steel,  tool-steel  and  cast  iron,  chemical  comparison  of  144 

Sulphur,  effect  of,  in  steel  making 150 

Sulphur  in  cast  iron 127 

Sulphur,  introduction  of,  into  iron 43 

Sulphur,  removal  of,  in  puddling  process 56 

Tar  as  a  binding  material 36 

Taylor- White's  high-speed  steels -  174 

Temperature,  control  of,  in  Bessemer  converter 63 

Temperature,  limits  of,  for  high-speed  tool-steel 174 

Temperature  produced  by  combustion  of  hydrogen 7 

Temperatures  in  engineering  and  metallurgical  processes 10 

Temperatures  resulting  from  combustion 3 

Temperatures  of  solidification  of  steel  and  cast  iron 75 

Temper  graphite 69 

Tempering  steel 167,  169 

Tensile  stress 92 

Testing  materials 91 

Thurston's  experiments  on  ternary  alloys  of  copper,  tin  and  zinc  189 

Tin 84 

Tin  and  copper,  equilibrium  diagram  of '. 183 

Tool-steel  and  wrought  iron,  difference  between 56 

Tool-steel,  cast  iron  and  structural  steel,  chemical  comparison  of  144 

Tool-steel,  cementation  process  for  making 57 

Tool-steel,  crucible  process  for  making 57 

Tool-steel,  sulphur  and  phosphorus  undesirable  in 151 

Tool-steels,  composition  and  cutting  speed  of 175 

Tool-steels,  high-speed , 173 


INDEX  215 

PAGE 

"Tossing"  process  for  tin 86 

Total  stress 92 

Toughening  steel 167,  170 

Troostite 166 

Tropenas  converter 73 

Tungsten,  addition  of,  makes  steel  hard  and  brittle 152 

Ultimate  resilience 99,  104 

Ultimate  strength  of  material 95 

Unit  stress 92 

Valves  for  engine,  material  for 200 

Vanadium 153 

Vanadium-chrome-nickel  steel 154 

Vanadium-nickel  steel 154 

Vanadium-steel,  composition  of 153 

Water-gas  process 21 

Wet  puddling 54 

White  cast  iron 122 

Wohler's  experiments  on  repeated  stress . 159 

Wrood,  composition  of 14 

Wrought  iron  and  tool-steel,  difference  between 56 

Yield  point 97 

Zinc 86 

Zinc  and  copper,  equilibrium  diagram  of 178 

Zinc,  Belgian  process  for  smelting 87 

Zinc  blende , 86 

Zinc  carbonate  ore,  roasting,  to  remove  moisture 87 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 

AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


OCT.,  20  1935 


LD  21-100m-7,'33 


re  to705 


321439 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


m 


